There are basic jump drills on varsity.com
Go to
www.varsity.comSelect the "Library" tab across the top of the homepage
Select "Skills and Drills" on the top row in the middle
Finally, you'll see the "Jumps" article with lots of advice.
Flexibility and stretching will help jumps. Don't forget that your abs do a lot of the work. Sit-up work your upper abs, but don't forget the lower ones. I suggest starting with the drills on varsity.com--see if you find improvement after a week or two of committed drills and stretching.