If you want to get scientific about it, you can measure the surface temperature of your pan with a handy dandy IR temperature gun (hat tip: Alton Brown), giving you the sort of precision that recipes which call for "medium heat" will simply never deliver.

I just got one and I'm hoping to sort out the precise conditions under which my crêpes either come loose right away or stick to the pan. Temperature definitely plays a part here. Also, my first crêpe *always* sticks to the pan, while subsequent ones don't, implying perhaps there's some transfer of butter (and/or egg, flour) from crêpe #1 to the pan that's helping subsequent ones.