I have found Liquid Wrench to be very good for loosening up rusted parts - it seems to work better for me than WD-40.
A dremel with a cut off disc can sometimes just cut one small slot in the side of the nut (clear through) and that will do it, if you can get to it with the dremel disc.
Vise Grips and just twist the bolt off? Or, grind the head off the bolt? Somewhat extreme, but effective!
Neal