Practice your second service in a tennis match situation
If there is a line between an average and a good tennis player, then it is defined by his second service. Good players have a second serve that has enough ball rotation to enable them to hit a rather fast and still consistent second serve. When you have just acquired this technique, you will encounter one problem: In a match, players tend to execute this shot rather timidly, out of fear of making a double fault. But in order to create enough spin to make that ball consistent and not too defensive, you have to move your racket fast.
So how do you develop enough trust in that shot? It may work perfectly in practice when there is little pressure, but in a match you have a problem.
Here's the solution to that dilemma: Find yourself a match partner who agrees on a little change of rules (your partner might profit in the same way from that change). Both players have three serves at their disposal. First serve is your regular, rather straight and fast shot, then you use the slice or kick serve you want to improve for second. Knowing that you still have another chance to serve, you will be more relaxed and more willing to hit that shot with the racket speed that it needs to have enough spin.
After a while, you won't even need that 3rd serve any more because the 2nd has become consistent enough!