Saddle Question

[BleeckerSt_Girl]

How exactly do I measure my sit bones to know what size seat I need?

Try reading these two threads, perhaps they will help:
http://forums.teamestrogen.com/showt...uring+sitbones
and
http://forums.teamestrogen.com/showt...uring+sitbones

Again, not sure if this is exactly what your problem is. It's normal to have soreness of your rear sitbones when you are a brand new rider, but what you describe sounds more like the frontal pelvic bone pair, not the sitbones? -and you are not a really new rider anymore anyway.
Myself, I have extra wide sitbones and I find the x-tra-wide Brooks B68 non-padded leather saddles to be the most comfortable for me on all distance rides up to 70 miles so far. But I combine it with some good long distance (dense foam, but not overly puffy chamois) shorts.
You might also want to experiment with slightly changing the tilt of your saddle's nose up or down 1/4". A little adjustment usually translates into a big change in how it feels (for better or worse).
This is an interesting picture- is this the problem you think you are experiencing?:


I'm still sure that others will have some valid input here as well. I'm only putting forth one idea.