well heres my tip for the flat serve,
ball in hand palm up .its important your palm be parallel to the ground.
use your front knee for a guide and drop you hand down so your knucles touch your knee.now keeping a straight arm and using your knee as a guide place the ball straight up to the highest point you can reach with your racquet.now practise this technique and your serve will greatly improve, there is not much that can go wrong with this very consistant toss and it is the toss i have used since i was a juniour, possibly the best tennis advice i have ever reiceved.