Developing a consistently great serve takes a lot of practice. You will need to get acceleration and upward motion right to deliver a killer serve every time. The Momentus Ace Trainer can help. It uses a magnetic sliding ball that trains properly timed acceleration and upward motion. It requires the right technique to release off the grip and slide up the shaft and strike the end.
This tennis trainer improves motion for backhand and forehand shots too. The above video shows what you can do with it.