Do you have trouble putting your service toss in the same place every time? If you do, you can make your toss better by identifying the exact spot in the sky that you want the ball to go to.
Try this. Go through your service motion and stop at your contact point-- this place varies depending upon the type of serve you're hitting. Keep your eyes on the spot that you've just identified and toss to it. Looking at this spot gives you something to aim for, and it's always easier to hit your target when you can see it.