I haven't been riding fixed that long, but have recently gotten to the point where I don't have to brake on any of my hills, and I've always had decent spinning form.

Anyway, in addition to focusing on spinning the circle as mentioned, I relax. It's almost a meditation, ignore the pain type of thing. Tensing yourself, such as trying to "push your rear into the seat", as mentioned, is just going to make things worse.

Relax and try to spin just a little faster than the pedals.