Stretch your calves, so that you can bring your foot closer to your shin and strengthen your tibialis anterior (front shin muscle), which will ensure a tighter grip. This is the most important thing to do, but you can also:
- get a ball made out of rubber (for example: Urbanball)
- shave the hair off of the part of your leg where it comes in contact with your shin (or your entire legs if you feel like it) or just use a sticky tape like Rocky Mayorga

Also, it certainly helps if your ball and your shoes are clean, because dirt messes up the grip and it's a good thing if you have flexible shoes.
Good luck! ;)
you should really ask billy wingrove...he's insane at them! it's a tough trick to master...(major sarcasm intended here)
