Wheelies are like any other biking skill - practice. Balance or "manual trainers" can help with getting used to finding the balance point, but nothing will really be a substitute for just going out and riding. There are many "how-to" videos and whatnot out there. Whatever works for you is the "right" way to learn.
I'm always blown away by the guys who can lean way back, feathering the rear brake and dragging the other hand on the ground. Bernard Kerr does this a lot in his videos. Most of the time running clipless pedals (it personally scares the crap out of me when trying to wheelie when I'm not on flats...).