[chris vickery]

An old wrench turners trick is to try tightening old rusted bolts and nuts before trying to loosen them. You would be amazed at how often this works.
I usually try to soak stuff the day before as well with penetrant.
Alternatively, heat can be your friend, either heating it up and turning it off red hot, or, heating it up and quenching rapidly with water which breaks the rust bond.
I have used almost any and all methods to bust out a seized nut and bolt over the years and typically it is not a "one size fits all" approach.
As my father taught me a long time ago, start with finesse and ramp it up a bit at a time until you find what works. A bigger hammer is not always the solution but sometimes neccessary. Good luck!