I've learned to wheelie when I was a kid riding a Honda TLR200 Reflex. Anybody remember those?
Slow wheelies are more fun than fast wheelies because of the initial hesitation to bail like Cam said. I can do slow wheelies on my 250 better than my 500.
I've been learning to wheelie my KX with the extended swingarm. It looks pretty cool, my 15 year old cousin is showing me up on that.
Cam, the stoppies are pretty easy. To learn rolling stoppies, I'd get on a slight downhill and grab a handful of break to get the back up, then slowly let off to let the bike roll, then grab it again. I can get a downhill pretty good and with alot of speed on a streight. But, you can be sure, I've flipped alot. I'd rather wreak a wheelie than a stoppie.
Once you get it down it is not any problem. Again though, my KX500 sucks at wheelies and stoppies compared to my YZ250 or the KX250. Both stomp it pretty good. Except for those 90mph+ wheelies. Then the 500 rules.