i have done some reading on this recently actually, and got round to cleaning it but not releasing the nut yet. either way, to summarise:
clean up the bolt head and thread as much as you can using a wire brush on a drill. I bought this one and it was excellent, works much better than the pics suggest:
then, now this is where you have a choice, either soak with wonder wheels OR white vinegar. Apparently wonder wheels fizzes and does all sorts of wonderful stuff to remove the corrosion, but I don't have any and used white vinegar and in truth I'm not sure it did all that much - So go with the wonder wheels option if I were you. To apply it I took a sock, soaked it in <insert product here> and wrapped it around and left it over night.
next day removed sock and re-attacked with the wire brush. Then gave the bolt head, and then the thread, a few swift taps with lump hammer to (hopefully) break up any corrosion inside the bracket. Then spray with PlusGas, you can probably do a sock soaking here too if you want to. Leave overnight.
remove sock, where applicable, give it a few more taps with hammer and applications of plus gas. have a cup of tea/coffee/bovril. now attack bolt with a long breaker bar. if you're not happy with the force being applied you can try heating the bracket. be careful as if it transfers too much heat to the shock you'll ruin it.