Telling someone to "relax" is all well and good... but doesn't do much to actually aid you in the midst of the invasion. (Heheh... the word "invasion" is soooo visual for me. )

But if you actually "push" as if you were evacuating your bowels (not squeeze but push) you'll help loosen your sphincter and open up quite a bit.

When I detect resistance at my girl's backside, I remind her to push and it definitely helps.

I was myself reminded of this technique just this morning by my doctor during my digital rectal exam. (Not that she has particularly thick fingers... but then I don't find myself bent over a padded bench all that often either. LOL)