Maybe you're not twisting the throttle fast enough?
That's a mistake I did first. Make sure you don't just give it more gas, you have to snap the throttle open as fast as possible. The speed at which you do it is very important. And like many said, yankin on the bars is not a good idea. It's probably what's keeping you from succeeding, you can't have good throttle control if you're yankin' at the same time. I grip the gas with my wrist angled fully downwards and then snap it open so it's angled fully upwards to get the wheel up in 1st. But dont keep it there

once the front is in the air or you'll go too far and crash. Baby steps.
I'll say it again, do not try to yank on the bars or use your muscles to lift the bike, it will only mess it up.
And move your ass back a bit, against the passenger seat, grip the tank hard with your knees so you won't move forward without noticing. Also, making sure you put your weight on your ass is much easier/possible at all only if you're not yankin on the bars at all.(I guess that's the third time I'm saying that)
Also try it going slightly uphill, it comes up easier.
While it's easy getting it up in first I find it's hard to keep it there, 2nd gear is much easier to achieve balance and that means either switching gears (which I haven't learned to do yet, I'm a total newb myself) or clutching it up which after a little practice is not hard at all. Be careful though, the rear tire starts to skid very easily and the bike comes up pretty fast, at first these seem to happen with so little difference in timing, throttle position, starting speed, body position, incline on the road etc. And while it's a cool feeling first having the rear skid and then pulling a wheelie you probably won't be going straight ahead anymore.
I find it's easiest if I do it like this:
Get going in 2nd to about 4000-5000rpm
Give it a good deal of gas *after* which, not at the same time, use 1 finger for the clutch, which is just a quick in'n'out thing and voila, there it is. Up, easily controllable and loooong wheelies.
But I definitely recommend learning 1st gear before trying 2nd.
Also, if you're practicing going back and forth along the same stretch of road and it's uphill/downhill, don't practise both ways, the difference between uphill and downhill will upset you learning the timing and throttle control.
Don't look *too* closely at other peoples rpms n such, bikes&divers differ, experiment, find out what works for you.
Like I said, I'm a total newb myself but thought I should offer a fresh perspective on learning wheelies.
If someone more experienced could give us tips on how to change gears it would be great

Do you get it up in 1st and immediately switch? Do you use the rear brake and if so when and how? Continuously or apply when needed?
cheers and let us now if you succeeded. It does take some practise, remember that.
Freedom is won with blood and lost with the stroke of a pen
--uknown
edit: If I'm confusing, check Creeps reply a little ways down