This is an experiment for Christians (or theists in general) who are in a crisis of faith. Perhaps you're hanging around atheist areas of Usenet or the Web because you've begun to doubt your faith, and are hoping to find something there to push you one way or the other. Well, I think I have it.
Think about this: Does God care whether you remain a believer, or is it all the same to him whether you believe or not? Most Christians would state that God cares very much, and would like as many people to come to believe in him as possible. However, if you've begun to doubt, you probably have some respect for the scientific mindset, and the idea that theories need to be testable. The theory that God cares about whether you believe would seem at first to be hard (if not impossible) to test, but if you recall that God is (allegedly) a thinking, intelligent being, it becomes possible.
Since God is also (allegedly) all-powerful, it shouldn't be hard for him to give you a simple sign (i.e., a miracle) of his existence which you specifically ask for. In fact, there are numerous places in the Bible where people asked for a sign (sometimes very specific) and got one (sometimes repeatedly). Although theologians and the Bible itself make contradictory statements on whether it's OK (i.e., moral) for you as a Christian to ask for a sign in the first place, no true believer really questions that God is able to perform miracles, and that he has sometimes done so at the request of believers (even one actively in doubt).
OK, so you think you could possibly ask for a miraculous sign. "God, please do such and such if you exist." Unfortunately there is a problem with this approach, one that theologians will only be too happy to point out: you don't tell God what to do; he's God! They will point out that the sign not being given could mean one of two things: 1) "No." or 2) "Later." Trust us, they'll say, God knows best, and if he doesn't do what you ask, who are you to question his motives? Silence on God's part doesn't prove that he doesn't exist.
First let's deal with the second response: "Later." This, you are told, is a way to build patience, waiting for God to grant your request or release you from your trial. But you're in a crisis of faith, remember? You don't have your whole life to wait around to conclude that you are, or are not, a theist. If you did, and at the very end decided that you were atheist after all, you will have missed many opportunities for a fuller, happier life without the carrots and sticks of theism hanging over your head. The way to put your request is to make sure there is a time limit. Yes, that's right: give God a deadline. The theologians laugh and say, "Or what? Are you going to cry if God doesn't do it on your schedule?" "No," you say, "I simply won't believe."
Here is where you put teeth in your request, and this also deals with the first response: "No." The problem with just asking God to please do something is that it leaves your interpretation of the silent response undefined. (In a speech he gave before the Society of Seperationists in 1976, George Smith also pointed out the importance of defining both responses when dealing with religious experience.) Atheists will point out that a third alternative to No and Later is that there is nobody to give an answer in the first place. A fourth alternative is that a higher being of some kind does exist but doesn't care enough about whether you end up a believer to bother answering (or even wishes you would stop believing, perhaps!) By defining beforehand what you will do for both a positive response (i.e., the miracle happens) and for a negative response (i.e., nothing happens), you set yourself free from not knowing how to handle the negative response. You are set free, because God (if he exists) knows how you will respond in each case and therefore knows how his doing, or not doing, the miracle will be interpreted. It puts the responsibility for the outcome squarely in his court.
For example, you might take an old towel from the closet and your wristwatch, and go out to the back yard or an empty field alone one night. (You can do this with friends, but they will have to be very understanding and supportive friends.) After picking a spot and removing any debris, you lay the towel down flat and then stand (or get on your knees, or whatever) about five to ten feet away from it. You then pray: "Lord God, if you exist, this is your final notice. They say you know my heart. If so, then you know I doubt. I am ready to live with you or without you. But they say you want me to live with you. Prove your existence to me by completely consuming this towel with fire within five minutes after I finish my prayer. If you do this, then I will believe in you without reservation, and will live my life as your servant. If you do not do this, then it will be clear to me that you either don't exist, or don't care enough about whether I believe in you or not to lift a finger to convince me, and so I will live my life as a nonbeliever: an atheist. They say you know how I feel. If so, then you know I am serious about this, and that I will do exactly as I say. Amen." Take a deep breath, look at your watch, and wait. Five minutes later, you will have your result.
If your result is positive, then it is settled: you are a theist. If your result is negative, you are now an atheist. Welcome to the club.
It's important to note that only doubters can make this work, because God (if he exists) knows that a true believer doesn't really require a miracle, and a true nonbeliever would not bother to perform this test in the first place.
It's important to note that no matter what the outcome of your run of this experiment, you will only convince yourself. Don't bother writing to me and telling me your miracle did happen, and therefore I should believe too. Your miracle is my anecdote. As long as you don't proselytize, I'd be interested in hearing your story about how your "challenge" went.