Slipping the clutch, involves pulling in the clutch a touch untill you get to that slip zone, giving it enough gas to raise the revs to desired level (somewhere around 6-12k) and then simultaniously letting go of the clutch and giving it lots of gas to keep from stalling.
It is the best way to bring up a wheelie in that its most consistent. However, the bike will jump up in a hurry, so its also an easy way to flip it. Keep that foot on the rear brake.
It is my personal opinion that wheelies should be done at the lowest gear possible. The speeds are lower, so if you mess up, its not going to hurt as bad. Further more, what is the point of reducing your final drive (sprockets), and then going up gears, when in the end you end up with the same over all gear ratio as staying in first gear with stock gearing?
The secret to a long wheelie is balance. If you can bring the bike up to balance point, you shouldnt be accelerating, and thus can keep the wheelie going for as long as you wish, without running out of revs. However, thats pretty difficult.
The second recourse it to shift during a wheelie. It needs to be a clutchless shift, which you can practice on both wheels. Simply put some pressure on the shifter, then in one motion, close the throttle and open it back up. When the tranny unloads, the shifter should slip right into next gear. So what you can do, is bring the bike up in first, right as it come up, shift to second and ride it out, or maybe even shift to third afterwords.
However, keep in mind that you dont want to be going 100+ on one wheel, its going to really hurt if you mess up.
For best advice, you may want to ask roadstainr6. He's the expert stunter in these parts