From the FAQ's section of wakeworld...
Q. How do I get bigger air off the wake?
A. An important element to getting more air has a lot to do with edging all the way through the wake. A common problem is that as you approach the crest of the wake, most people ease up the pull and flatten out the board- that is the worst thing you could do and here's why. By easing up on the pull, you take away most of the tension the rope has built up and significantly reduces the force generated that snaps you into the air. This is what is referred to as "loading the line"- building tension on the rope with the pull, your body, the board, everything! Then once that tensions releases, that force springs you higher and further to more of what you are looking for - BIG AIR! Next time you go out and try for bigger air, remember that a rider's maximum speed should be at the TOP of the wake- not as you first make the cut or anytime in-between. The best approach is to start with a gradual turn into your carve, dig that edge in as you build up speed toward the wake, accelerate all the way through and hold on until you get lift-off. Don't forget to spring off the top of the wake by extending your knees- this additional downward push on the board right before you leave the wake creates more lift and the kind of "pop" that generates better air. Might take some getting used to, but once you put all these together and work on the timing- you will be amazed!
Q. What's a good boat speed for Wakeboarding??
A. Generally speaking, beginners start out at around 17-18mph... that's a good speed to become familiar with the wakeboard while learning to control and manuever it. This speed is also slow enough that you don't experience much drag yet fast enough for you to practice cutting over the wake and moving into surface 180s and surface 360s.
As you improve and feel more confident, you can speed up to anywhere between 19-22 mph . Keep in mind, a lot of this is dictated by your inidividual size, the boat's wake, rope length, and of course your comfort level - also depends on how many people you have in the boat too! But it doesn't hurt to experiment and pay attention to what other people might be trying. Once you find that "sweet spot", that's about where you want to stay.
And finally, read this article and then go out and practice! http://www.wakeworld.com/Articles/2001/Basics.asp