What would make my rear main seal go out as soon as its replaced?

I have a 1995 Firebird 3.4L and I replaced the rear main seal twice and as soon as I start the motor it starts leaking. I have checked for burs and divets in and around the seal's surfaces and don"t see anything. The motor doesn't feel or sound like it is out of balace. The oil level is good. STUMPED!