Depends entirely on the kind of shake you're trying to get! Hehe 😉
The trick to what I consider the "normal" shake, or the bellydancer shimmy, is that you need to squat down just a little bit, and then it's all in the knees! You move your knees opposite each other, front and back (so when one is forward, the other is back) and very quickly! You don't need to do it very big, just enough to get all that hip/booty going!
Some people also do this with their feet...I'm not entirely sure how, because I imagine that would mean a lot of clattering when wearing heels! However, there's an excellent video of it. It's Michelle L'amour's performance at the 1st annual burlesque festival in new orleans! Yes, she's a burlesque dancer...and there's a reason her tag line is "The ass that goes POW!"
http://www.youtube.com/watch?v=mU2y_XVgd_A
She starts around 4:00 or 4:10, if you want to skip the delicious tease and get right to the bum.
She also showcases a couple of other ways of doing it, with some strong muscle isolation (by flexing her thigh muscles in the back, she can make the whole thing shake!), and the nice hip movements which allows it to shake as well!
And, I know that Veena has some wonderful lessons on booty shaking, if you've got access to her lessons.
And, if you REALLY need more help, take a little belly dancing or burlesque class! They'll help you out! 😀