Start with your weight on your back foot and you'll put the toss in the right spot.
PROBLEM
Your service toss is hard to control and tends to drift behind your head. This prevents you from getting forward momentum into your swing and makes it difficult to hit the serve with pace and control.
SOLUTION
When you’re in the set position before you serve, start with your weight on your back foot. As your tossing arm goes up, shift your weight forward onto your front leg. This will help you get your toss in front of your body. It will also get you leaning into the serve and your body moving into the court as you swing through the ball. With the ball in front of you, rather than behind your head, you can hit a stronger flat or slice serve. Initially you may feel like you’re falling forward, but with some practice this momentum will add even more power to your serve.