Both of my labors centered in my thighs. During my second pregnancy, I tried to prevent that by doing yoga, lunges, visiting the chiropractor regularly, etc. It didn't seem to make a difference.
I found standing and swaying during contractions most comfortable. Having a contraction with pain in my thighs while sitting on the yoga ball or kneeling was scary, which didn't help with relaxation. I also got in the birth tub towards the end with my most recent birth and just kind of draped myself over the side with my legs stretched out behind me. It didn't completely make the thigh pain go away, but it did seem to help.
My DD was born on the birth stool, and I don't remember if that was much better or not. I tried side-lying as well. It was tolerable, but the thigh pain didn't go away until my babies were born.
I read a book that described a vigorous thigh shaking technique in labor--I wish I could remember the title of that book. Basically, your birth partner and/or attendant jiggle (and not lightly, but firmly) your thighs during a contraction to loosen the muscles. We tried that when I was in the birth tub and it helped a bit.