A good way to practice your serve rather than slaming the ball down into the corners of the servie boxes, is to put targets half way up near the doubles lines(O) and practice that so important lateral serve.

You can also use something outside the doubles lines, a marker (T) or just focus on a point on the side netting/fence whatever you have, and see if you can get the ball to bounce for the second time near that point.

Remember the second bounce point will be where your opponent has to be to receive that is important.

If you want to practice the serve down the middle of the box, then make it your practice to look for the second bounce outside the baseline, and so keep your opponent deep.