I just posed this same question to a wonderful dancer that I had the priviledge of hosting a workshop for this past week-end and also of attending her performance last night, Sedona from Portland. She has THE most incredible shimmy I have ever seen. I couldn't take my eyes off her hips.
And I thought I could shimmy. Heh. I asked her this morning before she left...HOW do you shimmy with such power? First thing she said was "Practice! practice! practice!" She told me that she would practice shimmying at every opportunity she had, even while brushing her teeth.
She also said that it's very important to loosen everything up completely, until you feel that even your bones are loose.
She said the even a tiny bit of tension in the legs, hips or glutes will hinder your shimmies.
So, loosen up and practice! I know I'm going to.
Regards
Priscilla