Walking in the deep sand under the hot sun can be pretty tiring. If you jump and run in the sand for hours per day, obviously you need very good fitness. If run out of oxygen after a short sprint, you'll be unable to concentrate on even the simpliest drills. So, first you need to build up your fitness.
Here is a video about conditioning from Coach Cindy Phillips:
Volleyball Specific Workout on the Beach Part 1
Volleyball Specific Workout on the Beach Part 2
King of the Beach
