Get a good drummer that can play the drums properly.
It's the most common thing, for lesser quality drummers to hit the hat too hard and the snare too quiet.
If it's going to be an effected / squashed snare, listen back with compression cranked whilst tracking to see how much bleed is getting in.
Get the drummer to physicaly move the hat firther away from the snare, even an inch or two may help.
Pointing the rear of the snare mic at the hats is good for rejection, but difficult in practice. Put cans on when setting up the kit, get the drummer to play, listen to the snare coming back with compression on it, and move the mic about until you get less hat.
Or buy a drum machine
(only puch the information in once and all that)...
