I've had good luck drenching it in PB Blaster and then having a sandwich. Tapping it with a hammer also lets the stuff work deeper into the crevices.
I recall one bolt that I had absolutely no luck in moving. I drenched, waited, tapped, waited; then I put the wrench on it. It actually moved while I was reaching for the cheater bar. In less than a minute this rusty nut that hadn't turned in over 10 years was in my hand.
