From a post in my log about hip bumps:
While basic, hip bumps and scissors are my top sweeps and really all I get on guys that are better than me too. With the hip bump there are a few things that I do that I've never seen stressed that I think help make it work for me. First is the grip. I basically start with a kimura grip but then switch the vining arm to their wrist. This leaves me with the open arm that I need to get up and to load my hips. The control on their arm is actually pretty solid though and gives a lot of leverage you don't have with standard grips. From the overhook the other thing I stressed, which I think a lot of guys do even if they don't realize it, is really think like you're doing an over shoulder roll. If you visualize it that way you drive their arm across, under and through your plane. This can actually, assuming your hip movement is good, can actually allow you to get some pretty good momentum on the sweep.