I'm a 100% bottom guy, and I love really thick dicks (and I can take a really really thick one with no problem), but there are a few things I have noticed. 1) If I really like a guy and I get really nervous I get so tight that sticking a finger let alone a cock in there becomes impossible. 2) If I relax my abs my hole will also relax. 3) It helps with a really big one to push out as he pushes in. 4) If you learn how to control your sphincter muscles it helps. You have a lot more control over your ass than you think. You can open wide or squeeze so tight that you will hurt the guy your with (I have done that before). When I was young I used to do the original Jane Fonda work out (from the tape) with my aunt. There is a buttocks excercise that not only seems to work your butt, but I later learned it's great for your sphincters also. Using these excersices you can both expand and contract. I can take the largest donkey kong there is (and I love it rough and hard), but when I've had guys with little tiny ones fuck me they always tell me that I'm the tightest guy they've ever fucked.