Weights help tremendously. Here is a list of stretches and plyometric drills that will help to improve jumps that require for my team:
Sitting stretches (knees pointing to the ceiling, point toes!!):
Straddle stretch - 10 seconds
Right stretch (chest to knee) - 10 seconds
Right stretch (bow & arrow) - 10 seconds
Middle stretch (knees pointing up, roll hips back, point toes)- 10 seconds
Left stretch (chest to knee) - 10 seconds
Left stretch (bow & arrow) - 10 seconds
Middle stretch (walk it out further) - 10 seconds
Legs together (flex feet) - 10 seconds
Legs together (point toes) - 10 seconds
Right split (chest to knee) - 10 seconds
Left split (chest to knee) - 10 seconds
Kicks (3 reps):
10 kicks - to the right side, kicks should be to your ears, high V motion
10 kicks - to the left side, " "
Jumps (3 reps):
10 Tuck jumps
10 Spread Eagles
3 Toe touches
3 Right Hurdlers
3 Left Hurdlers
3 Right Hurkies
3 Left Hurkies
3 Double Whips
1 Triple Whip
Drills:
10 - (5 reps) Frog Hops (touch the ground, jump up with hands in the air)
10 - (3) Switch Lunges (lunge, jump as high as you can after every lunge, alternating legs)
Doing these 3x/week will improve your jumps. Oversplits, continuous stretching, partner stretching, etc. are all good advice as well!
Good luck!