Next time you go out to practice your serve, try this! Imagine a sidewalk, approximately two feet wide, on the inside of the perimeter of the service box. Pretend that the area not covered by the sidewalk is a water trap like in golf. So, every time you hit the water and not the sidewalk you lose that ball... or in a real game, pretend you would lose that point. This will give you a purpose with your serve.
 
Even if you have a weak serve, keeping it on the sidewalk will either press the receiver back or move them enough left or right to not let them plant their feet too long to kill the ball. Knowing where you are going to hit the serve, then accomplishing it, is incredible. Do this every time you practice your serve.
 
It is especially important in doubles that you, as the server, can protect your net player from being attacked if your serve is weak. This concept will work great if you have an offensive serve and will be the best thing you can do if you have a weak serve.
 
Good luck and have a great 2018 on the courts!