Divide both service courts in two halfs. The server has to serve one serve into each half starting from left to right.

a) He needs to hit four serves. Count how many he needs to complete the sequence.
b) He needs to serve two in a row in each half of the court. If he hit every serve, that would be 8 serves.
c) He needs to serve 3 in a row. That would be 12 serves if he hit every time.

Variation:
Make player serve in thirds of the court.

Benefits:
This drill is different from usual target practice drill because the player is under pressure. He needs as few serves as possible and he starts from 0 if misses a second or a third serve as mentioned in points b) and c).

He of course practices precision serving but he is much more focused in this drill than if he just practices target serving.