If I've messed some or you think you could word oneor more better, post it in a reply and I may add it.

1. Know what atheism is: Atheism is s spectrum of beliefs not a single belief. On one end you have agnostic or weak atheism on the other you have strong atheism; agnostic or week atheism is the lack of a belief in god or gods but not the belief that a god or gods couldn't or don't exist, strong atheism is the belief that a god or gods do not exist. Most atheists are somewhere in the middle, for instance me; when it comes to the gods of the bible, the god of Christianity,  Judaism and Islam, the gods of pagan religions, Hinduism, Shintoism and all other gods that people believe or have believed in, I believe they do not exist as the universe does not is not fit what it would look like if they or one of them existed, however a deistic god(s) (a god that creates the universe and does not reveal it self through miracle and revelation, a god(s) that hides it self or a god(s) that create the universe and is indifferent to us cause we are an unintended and uninteresting (to it/them) byproduct) I am agnostic about, theres no way to prove that such a god(s) does or does not exists.
2. People don't just convert: No single argument will convert anybody, there are very few situations where a person will convert; a life changing event, a traumatic event or through a long time of thought.
3. When an atheist says "I don't believe in God" he/she doesn't believe in God: Not every one believes in God, we are not in denial of our beliefs, we do not believe.e
4. Don't quote the bible (or what ever your holy book is): Unless the conversation turns into a debate about the bible (it has happened before, it will happen again and it is happening right now somewhere in the world) don't quote the bible. I understand that to you its some grand holy book of truth but to us its mythology, quoting a bible in a debate with an atheist is like a pagan in a debate with you quoting something Zeus said.
5. Don't say "you have to have faith": Faith is not a logical argument, saying "you have to have faith" is the same a saying "just believe, it doesn't matter if it doesn't make any sense, have any evidence for it or even evidence against it, just believe" It may work for those who already believe and see faith as some kind of virtue but for those who don't believe and don't see faith as a virtue it doesn't work.
6. Just cause your happy in your religion doesn't mean every one else would be: Every one is different, for many your religion my not make them happy, it may even make them depressed.
7. There is no such thing as a "true Christan": The only Christen who do not call them selfs "true Christen" are the only who do not use that phrase. All Christens (accept those who do not use that phrase) of all denomination call them selfs "true Christens" and they can not agree what a "true Christen" is. Christen A says Christen B is not a "true Christen", Christen B says he is a "true Christen" and Christen A is not and Christen C says he is a "true Christen" and Christen A and B are not. All People who identify them selfs as Christens are Christens, it doesn't madder if you agree with there version of Christianity or not.
8. Stalin and Mow do not represent atheism: Yes they where atheists but that has no baring on morality of other atheists and all them being atheists shows is that people of any belief system can be evil
9. Hitler was not an atheist: Hitler was not an atheist ^[1] Hitler was not an atheist ^[2] Hitler was not an atheist ^[3]
   And even if he was, see Stalin and Mow^
10. Recognise that you are flawed just like every one else: People of all religions "witness miracles" and people "see ghosts" "see UFOs" "experience alien abduction" and so on. Humans are fallible creatures, our memory is fallible, our experiences are fallible, out perception s fallible. It is easy to trick some one into believing something when they wont to believe, it is easy to trick your self into believing something when you wont to believe and people have confirmation bias. You can not dismiss the miracles of other religions with out dismissing your own, you can not dismiss alien abduction, people seeing ghosts and UFOs with out recognising that people are fallible.
11. Don't use Pascal's Wager: Pascal's Wager is "Why not believe? If its wrong you loose nothing but if you don't believe and God is real then you go to hell" Pascal's Wager make several false assumptions. You choose what you believe, you loose nothing by believing in God and theres only two options. Belief isn't a choice, you do loose things by believing, many people have lost many things, (harmless pleasures, there Sundays, what choices they can make and even there lives) and there is allot more than two options, Christianity, Islam, Hinduism, Buddhism and many more.
12. Do not say atheist are amoral or have no basis for morality: Morality is separate from religion, it does not come from a god or gods. It comes from social need, with out it we could not function as social creatures.
13. The fallowing apply to all arguments and debates
    1. Do not quote mine: Quote mining is taking a quite from some one and using out of context. Fort example Darwin's eye quote; creationists often quote mine Darwin by using this quote
       

       To suppose that the eye, with all its inimitable contrivances for adjusting the focus to different distances, for admitting different amounts of light, and for the correction of spherical and chromatic aberration, could have been formed by natural selection, seems, I freely confess, absurd in the highest possible degree.

       But they leave out the rest of the quote

       Yet reason tells me, that if numerous gradations from a perfect and complex eye to one very imperfect and simple, each grade being useful to its possessor, can be shown to exist; if further, the eye does vary ever so slightly, and the variations be inherited, which is certainly the case; and if any variation or modification in the organ be ever useful to an animal under changing conditions of life, then the difficulty of believing that a perfect and complex eye could be formed by natural selection, though insuperable by our imagination, can hardly be considered real. How a nerve comes to be sensitive to light, hardly concerns us more than how life itself first originated; but I may remark that several facts make me suspect that any sensitive nerve may be rendered sensitive to light, and likewise to those coarser vibrations of the air which produce sound

       Qute mining does not prove anything and only makes you look bad

    2. Anicdotes are not evedence: Anicdotes don't prove anything, you can talk about the "mericals" youve seen all you won't but that donesn't prove anything.
    3. Don't generalize: Each atheist is different we are not all the same we do not hold the same beliefs, ideas and values. The same goes for every other group.
    4. False statistics: Do not make up statistics, if your going to use statistics have an objective source for them.
    5. Fallacy's: these are fallacious arguments that should not be used
       1. Straw men: A straw man argument is a fallacious argument where you set up an argument that is a misrepresentation of some one else's argument (straw man) then debunk the misrepresentation. Example: Argument: With out religion I'm free of all the pointless rules that religion has. Straw man: So with out religion you have no morality, you have nothing to stop you from raping and killing. The argument was "with out religion I'm free of all the pointless rules (like not being allowed to pleasure yourself, not being allowed to be gay, not being allowed to say certain things, having to go through pointless rituals and so on)" it said nothing about morality or rules that are not pointless.
       2. False dichotomy: A false dichotomy is when you present an argument as if it only has 2 options when in reality there are more. Example: "Jesus was ether a lier, insane or the Messiah" this is a false dichotomy cause there more than those 3 possibility's, he could be completely mythological, the stories could be an amalgamation of mythology's, stories and historical people, there could have been a historical Jesus but his teachings could have been altered and exaggerated over time.
       3. Slippery slope: A slippery slope is argument that states that cause "A" leads to "B" and "B" leads to "C" and "C" is wrong thus "A" is wrong. Example: "If we allow same sex marriages marry then its only a matter of time before we allow people to marry animals and children"
          Whether "A" leads to "B" and whether "B" is wrong or not has no barring on whether "A" it true or not and often "A" does not lead to "B" cause if "B" is wrong then its wrong (same sex marriages are not wrong and marrying animals or children is wrong)
       4. Red herring: A red herring is a response to another argument that does not address the arguments. Example: Argument: There is no evidence for the existence of God. Red herring: You say theres no evidence for God but you can not disprove the existence of God.
          The argument is not about the lack evidence for the existence of God not about disproving the existence of God, the red herring avoids the argument about the lack of evidence for the existence of God and instead state that you can not disprove the existence of God which is a different argument.
       5. Ad hominem: An ad hominem (Latin: "argument to the man") is not an argument but an attack on some one and an avoidance of the persons arguments. Example: "How can you talk about right and wrong? Your an atheist, you have no morals"
          Ad hominems don't deal with the validity of the argument and thus say nothing about the validity of the argument.
       6. Argument from
          1. Authority: An argument from authority is an argument states that "X" authority says "Y" thus "Y" must be true. Example "The Pope says using contraceptives is a sin so it must be a sin"
             Just cause some one in in a position of authority does not mean that there are right, arguments must be based on logic and evidence.
          2. Popularity: An argument from popularity is an argument that states that cause something is popular it must be true. Example: "Every one uses Windows so Windows must be the best OS"
             Just cause something is popular doesn't mean its true, right or good.
          3. Ignorance: An argument from ignorance is an argument that states that cause we do not know the answer to something it mean that "X" must be the answer "No one know where the universe came from so God must have done it"
             Just cayuse we don't know the answer does not mean that "X" is the answer, it means we don't know the answer.
          4. Incredulity: An argument from personal incredulity is an argument that that cause the person doesn't know or can't imagine how "A" could be true, it must be false. Example: "I can't imagine the complexity of life could come about by chance, there had to have been a designer for such complexity to exist"
             Whether "A" is true or not is independent of persons ability to imagine how it could be true.
          5. Emotion: An argument from emotion is an argument that appeals to people emotion and bypasses actual logical or evidence and does not deal with the validity of the argument. For Example "If you don't believe you will go to hell where you will suffer for ever. Do you wont to go to hell" The argument appeals to a fear of hell, it doesn't deal with the validity of the argument.
          6. Nature: An appeal to nature is an argument that states cause something is natural or unnatural it is right, wrong, true, false, good or bad. Example: "Homosexuality is unnatural so its wrong"
             Whether something is natural or unnatural has no barring on whether it is right, wrong, true, false, good or bad.
          7. Consequence: An argument from consequences is an argument that states that if "X" is true there would be good or bad consequence thus "X" is false or true" Example: "If evolution is true than that means humans are just animals, we are not animals so its not true"
             The consequence of something being true or false has no baring on whether it it is true or false and these consequences are not false.

Update 8/9/08: Added "When an atheist says 'I don't believe in God"'he/she doesn't believe in God"
Update: Added "There is no such thing as a 'true Christan'"
Update 8/1/08: Added argument from personal incredulity.