Tips for Tennis?

I'm doing tennis, and I'm okay with a forehand & backhand & volley. But I really mess up on overhead serves! My instructor tells me that I should "slow down and toss the ball correctly". He says to let go of the ball (when I throw it upwards) and it's at shoulder level. I've improved a bit on that, but of course I'm searching for any more tips on how to do it. Also, advice for the other types of serves, forehands, etc., is accepted gladly!