I am wondering about how far back to shift my weight when going downhills on my mountain bike. I sit way back (actually squatting), but I'm never fully past the seat. My inner thighs keep me from going past my seat in the back. Is this ok? I've heard people say you should almost touch the back wheel with your butt, but I really have to spread my legs for that, and it just looks and feels silly.
I don't understand "inner thighs keep me from going past my seat" I don't exactly have toothpick thighs and I have no trouble. Are you gripping with your thighs? You might need to loosen up, a lot. Don't worry about feeliing or looking silly... a good rider is loose and moves their body around a lot. You can't have a tight knee/thigh grip and get back.
Look at this photo, you can see the seat in front of my crotch if you look hard. As I went down that, you better beleive that my butt was back behind the seat. You just don't want to touch the tire with your butt, that's very bad. Now, this is pretty steep but you get the idea.

To practice, get out of the saddle into the attack/neutral position. Practice moving forwards, backwards and side to side. Let your legs loosen up a bit. Have someone hold the bike still by the handllebars and practice moving back, put that seat on your tummy. Do check your saddle, if you have a "comfort saddle" it's definatley too wide.
2015 Liv Intrigue 2
Pro Mongoose Titanium Singlespeed
2012 Trek Madone 4.6 Compact SRAM