5'9'' here and i can DUNK!
(a tennis ball)
but this also requires full basketball gear = shorts, light tshirt, and shoes...
but anyways...try heel lifts...where you stand on your toes for an amount of time or for however many reps...
if you can do it with extra weight (carrying a barbell, weights, backpack, holding a chair, whatever)
dunno how much you gain from it, but it helped me out...