To do a better over hand serve, you should sorta flick your wrist. It always works for me and I am one of the top overhand servers at my school. I also grunt when I serve, just because it gets me ready, and it terrifies the other team. My coach also says on your toss up you should follow up with your arm slowly, and then at the peak of the toss, explode your arm. Works for me every time.
To stretch, you should concentrate mainly on your arms because they are where the main action is. (But do stretch your legs) Just some 15 second group stretches are good for me. As for warmups, we always would run 2 times around our side of the court, then on the 2nd time around we would jump up to practice blocks. Then we would practice passing where the coach would serve a hard one, and we would go for it one at a time. Then we would pepper, which is where 2 players are partners and one tosses it to the other player, that player passes it up, then the other player sets it, then the other player hits it, then a dig, and it keeps on going. We would also have our setters up at the net, then the coach would toss them the ball, they would set it, then a hitter would go hit.
This is what my team would normally do, and we won 1st at a tournament.