I have noticed that many people, both non-theists and theists, make some serious errors when debating the existence of God. In light of this, I will just mention a few that seem to be the most common, point out why they are wrong, and offer advice for correction.
1) It seems to me that each time I discuss theism, somehow arguments come up regarding the difficult passages in the Scriptures. You know the ones that I am talking about: those regarding slavery, God commanding war, etc... Now, I am not writing to say that these are trivial matters. I believe that they are difficult questions that require an honest hearing. However, they are irrelevant and have no place in the debate over the existence of God. When an atheist/agnostic/skeptic brings these issues up, they are only introducing a Red Herring and trying to win an argument with an emotional appeal. This is an intellectually dishonest approach. Now, don't think I am trying to say that only non-theists do this, many theists try to use the arguments from Hitler and Hell in their debates. This is just as dishonest.
2) Another common mistake is related to this one, it is going off topic. I have just allowed this to happen in my most recent blog conversation. We were debating the existence of a first cause, and before I knew it, I was defending the design argument. I don't know how this shift occurred, but it did. Again, I do believe that, as theists, we must be able to answer the questions leveled against our view, but we only have to take one at a time. By answering one at a time, this allows you to close the door on successive issues and to avoid having to give multiple simplistic and unacceptable answers to good questions. This often happens in debates between theists as well. Stay on target and the truth will weed itself out when good questions are brought forth.
3) Next, and this is the one that most annoys me; make an attempt to understand the other sides position before you criticize it. Don't build a straw man. I wish I had a dollar for every time that a non-theist criticized the cosmological argument because it doesn't account for a cause of God. It does not have to. The arguments first premise says that all things that begin to exist must have a cause. God did not have a beginning and therefore is not subject to this argument. Another straw man I have often seen is when someone confuses an infinite being with an infinite number of things. These are two different concepts, stop equivocating. Now, I know it seems I am picking on non-theists here, so I want to be clear that theists do this as well. I am not as familiar with the common (and annoying) straw men put forth by them, so if anyone wants to post some, I would be glad to see them.
4) Finally, know the arguments that you are presenting before you present them. Some people use weak arguments to try to prove their point. One is the argument from experience. I am sorry people, but your experiences are irrelevant to me. You cannot transfer them to me, so I cannot participate in them. Give me reasons. Another type of weak argument is one that has been shown invalid (for its purpose), such as the ontological argument, David Hume's argument against the miraculous, and Pascal's Wager. Let's take Pascal's Wager as an example. Pascal's Wager says:
"God either exists or He doesn't. Based on the testimony, both general revelation (nature) and special revelation (Scriptures/Bible), it is safe to assume that God does in fact exist. It is abundantly fair to conceive, that there is at least 50% chance that the Christian Creator God does in fact exist. Therefore, since we stand to gain eternity, and thus infinity, the wise and safe choice is to live as though God does exist. If we are right, we gain everything, and lose nothing. If we are wrong, we lose nothing and gain nothing. Therefore, based on simple mathematics, only the fool would choose to live a Godless life." Let us see. Since you must choose, let us see which interests you least. You have nothing to lose. Let us estimate these two chances. If you gain, you gain all; if you lose, you lose nothing. Wager, then, without hesitation that He is.
Many theists like to use a version of this, but is it valid? No. First, it equivocates on God. What is meant by God in this argument? According to Christianity, it is not a safer bet to believe in Allah or Buddha. Furthermore, what does he mean by believing "we lose nothing" if God doesn't exist? I say that we lose everything by believing in a non-existent being; especially Christians. Christians often dismantle their entire lives (at least if they are living consistently with their belief system) and forsake many worldly pleasures based on their beliefs. Now, if they are doing this for a God that doesn't exist, then they are throwing away everything, they literally lose it all. I have been on the proverbial "other side of the tracks" and admit that it is often more fun. I would not believe what I do for the sake of believing in something, I believe it because it is the most rational option and is historically verifiable. Know your arguments and their flaws and be prepared to defend them, though only one at a time, of course.
Feel free to add common errors that you think deserve a mention. By no means do I think this list is exhaustive.