Re: The serve
A big tip that I know a lot of people suffer from for not knowing, is comming through on your serve. Do not turn your shoulders all the way to where you want to hit, instead always picture having your shoulders parallel to the net when your finished.
A lot of instructors also say this: think like your throwing a basebal, in order to get the idea of fluidity, but it is a true statement. One thing you can try for more power is to slow your racquet speed down a bit while your in mid pick-up, and then accelerate quickly through the rest of the serve while still maintaining the proper motion. This is a way to cork yourself up into a position where you are ready to explode into the serve, and thus your get more power, and I find it easier to maintain and control over a long match than a continually accelerating motion where you have to expend a lot of energy and still time it correctly in order for it to work. Most good servers (Phillippoussis, Federer, Sampras, Safin) have a favorite time in their serve (Usually right before the racquet gets above the shoulder) where the slow the racquet down slightly before exploding into it.