Target Your Toss for a Consistent Serve
One of the keys to a consistent serve is a consistent toss. There are a couple of simple reasons for that. First, for every location the toss goes, that's a slightly different serve you have to execute. Second, if you don't have control of your toss, you're adjusting your motion in mid-stroke to accommodate your toss.
If you are able to toss the ball accurately to a target location, you'll be able to execute the same stroke every time. By being able to use the same stroke, the repetition will give you more confidence in your serve and the muscle memory will help you successfully execute it in pressure situations. So many people just toss the ball up any old place and go chase it, and they wonder why their serve is so inconsistent.
If you watch the pros, the best servers always know exactly where they are tossing the ball, so they execute their serve with the intention of making contact with the ball in a particular spot. Sure, there are some adjustments for things like the wind, but for the most part, they know where the ball is going to be when they swing. In fact, many pros look up to the target even before they toss the ball, as you see Vania King doing above during a first round mixed doubles match at the 2006 US Open.
When you practice your toss, aim for a target in the air, and catch the ball rather than chasing it if it's out of the target area. As you get used to your toss being where you expect it, you'll notice your serve getting more consistent.
One final note is that this tip isn't to say that you should always toss the ball to the same spot. While it's good to place your toss in the same spot for disguise, while you are learning different serves you'll find that different serves are easier with different toss locations. So as long as your toss is consistent for that particular serve, you'll be able to hit the serve with confidence!

