Brake control is one of the keys to wheelies. If you feel like you're about to fall backward, give the rear brakes a tap to lower the front wheel a bit while you find your balance point. But, be careful not to press the brakes too hard, because it will cause your front wheel to drop all the way back to the ground. It's a mixture of tapping the brakes, using your cranks, and balance. Watch this quick video from Dblocks on his Dblocks Big Ripper to see brake control in action.