There's nothing like putting some targets down in the service box for working on your accuracy.
This very simple drill involves hitting sets of ten serves at a single target. You should aim to land seven of your ten serves in the box, with three of them hitting the target.
Do the drill three times, with one target in the T, one in the middle of the service box, and another out wide.
If one of the targets is too hard for you to hit, then try to hit a certain area instead.