If you have a strong first serve and a tentative second serve, there are two things you can do. You can accept your weakness by being more consistent and less explosive with your first serve, so you don’t have to hit as many second serves. Or, you can dedicate yourself to improving your second serve—which will make your first serve even better, because you can go after it much more often.
How do you improve your second serve? First, remember that arm and racquet-head speed are critical to a good second serve. To quicken your pace, walk to the service line and hit second serves. In order to land the ball into the service box on the other side of the net, you’ll really need to generate racquet-head speed. Once your shots start going in, progressively move back to the baseline and do the same thing.
Another way to boost your second serve is to shorten your swing. I did this with Mary Joe Fernandez at the end of her career. Rather than taking a full backswing, she took a half backswing, which got the racquet to move faster, therefore putting more spin on the ball.
You can also do what Boris Becker did. At the end of each practice, he would hit serves to different spots of both service boxes—10 serves to the center, 10 to the left, and 10 to the right. Doing this will develop good habits and give you more confidence.
Above all, you cannot be afraid to double fault with your second serve. If you just push it in, you’ll never get better.