Shaun Sharp born in Heidelberg, Australia ;-), working  as tennis coach at the Meehan Tennis Academy in Melbourne, Australia, describes an implicit drill do improve and develop the service:

„I’ve been doing a serving drill where players serve from various positions in court, to varying targets….normally not to service boxes, and allow the students to work out how to serve ball to each target….
Via exploration and some visual or verbal instruction the students are able to discover how to best achieve the goal that is set.
It enables the kids to practice the flat serve, slice serve and kick serve without any explicit instructions….and kids are able to solve the motor problems themselves.“