Practice makes perfect is we practice perfectly. But, perfect practice is a tricky business. The following article from Vic Firth outlines very effective practice tactics for drummers. I've added jazz piano addendums in brackets.
In a nutshell:
a) Warm Ups (some sight reading)
b) Rudiments – learning and application of (jazz chords, modes)
c) Groove & Fills (working on your jazz licks through transcription)
d) Learning a song (using a combination of lead sheet and recordings)
Why study in the summer?
You've got to learn your instrument. Then, you practice, practice, practice. And then, when you finally get up there on the bandstand, forget all that and just wail.
I'm a professional pianist and music educator in West Toronto Ontario. I'm also a devoted student and teacher of the drums.