The biggest thing is getting over the fear of going over backwards. In order to ride wheelies, you have to find the balance point (as mentioned). In order to do this, you need to point the front end of the quad way up in the air, which requires you to be looking at the clouds. I don't know about the rest of you guys, but this was a big issue with me at first. I don't like the feeling of falling backwards, so I struggled with the whole wheelie riding thing. What help me get over it was to try it at slower speeds in first gear (which helped with engine braking which made using the rear brake less critical) in tall grass rather than hard-pack dirt or pavement. Going over isn't fun on any surface, but it is more forgiving doing it on tall grass versus pavement.
One of the things that will make things go much easier is your position on the quad. The further back you are on the seat, the easier it is to get the front end in the air. This is also a much more controlled way to do it versus using a lot of throttle and trying to sky the front end with brute power. This is especially true with guys learning how to wheelie. First learn body position and balance. Once you have figured out what you can and can't get away with as far as balance and body position, then you can move on to lofting the front end using power. If you first try riding wheelies using brute power, you will be moving faster which is going to be that much worse if you crash. Start slow, learn proper body position, balance and rear brake control and go from there.
Oh, do I really need to mention to wear your gear?