Many players have at least one strong serving motion. Often they are forehand pendulum (racket tip down), forehand tomahawk serves (racket tip up), or standard backhand serves with a left-to-right motion (for righties). These are all excellent serves that opponents at your level will inevitably adjust to. Perhaps watch how coaches or top players do it. (Youtube is your friend.) Or just ask them to show you, and they’ll be delighted.

To make these serves more effective you should develop the reverse versions. They might never become equal in effectiveness as the regular version, but at the very least they give you a variation that the opponent isn’t used to. This makes them uncomfortable, plus they now have to guard against these serves as well as your regular versions. Plus, some players (including me!) are better against one type of sidespin serve than the other.

 Here’s a simple way of looking at it. Have you ever found a player who was happy his opponent had more serve variations? So, be perverse and do the reverse!