One little trick that nobody has pointed out yet.......If your going at a slight grade uphill then the front tire is way more prone to lifting off. It all has to do with the center of gravity and balance position. If anyone has ever tried doing a wheelie going the opposite direction (downhill) then you know what I'm talking about.
So, in short, if your new to wheelies, try it out on a slight incline and you'll be suprised how much easier it is to lift off that front tire.
Don't forget to wear your cabage protector!