[Jacques Reed]

All very good methods mentioned here. Agree that heat is often the best method where safe to do so. Even before I had oxy-acetylene I have had good results with just the good old Bernz-O-Matic torch.

Another couple of tricks to add to the repertoire is to rap the top of the studs a few times before attempting to back out. Again this is all about breaking the rust bind with slight movement at the threads.

I have also had some success with a Dremel tool and a small cutoff wheel cutting away the side of a wasted nut. Can do the same with a hacksaw if access permits.
Have also used a "Gator Grip" movable pin wrench mentioned above and was successful with it.

It is always best to remove as much rust and dirt from exposed threads before starting to disassemble things. Penetrant works better when they are clean. Wire brushing is a good start and where possible I sometimes have immersed exposed threads in weak phosphoric acid (paint prep deoxidizer) to clean threads completely.

As others have mentioned here, patience is the key and trying different methods depending on the situation will usually result in success.

Cheers