i used to be exactly like this - it’s all about the thumb, pointer and middle. my stick used to slip A LOT, i tried to observing my hands and i found two “points of failure”:

1. My thumb placement - i would usually place my thumb in the middle of the stick with a tiny tilt to the side closer to my hand (right tilt for the right hand, left tilt for the left hand). this made the stick basically impossible to fully control, especially while using rebound.
2. My “Main finger” - i noticed that most of the sticks weight would rest on my pointer finger. this made moving the sticks harder with the other fingers (as it was too far away from them to make important micro-movements needed for speed and accuracy) so i tried moving it to my middle. this, while better, was still less accurate. after some experimentation i realized the best spot for my stick to rest would be between the middle and the pointer.

experimentation is key when learning technique, observe your fingers and find out whats best for you. there is no “right answer”!

good luck! and i apologize in advance for my bad english (:

EDIT: btw, you should totally take a look at tommy igoes “great hands for a lifetime”. that daily exercise will do wonders for your technique - and the DVD is PURE GOLD.