Penetrating fluid of some description (any of the above, or plusgas -- but NOT WD40...)
Do the screw up a touch first until you hear a crack or it moves a little - it's much easier to crack off the corrosion that way. Then don't try to unwind it all at once, come out, then in, then out a bit more, then in, etc... Keep applying the lube.
You may need to hit it. Although if you haven't used impact to free an electrolytically corroded bolt before this might not be the very best thing to practice on.
If it does end bady and the screw snaps use a left hand twist drill slowly with plenty of lube - don't go anywhere near those godawful 'ez-out' things - those that are like a tapered left hand thread screw. Odds on that will snap as well and then it's even more of a ballache job. Oh, and of you have to do that I'd probably say to do it in-situ otherwise the studs will shear off the mounting rubbers and you suddenly have another job and an extended 'parts to buy' list.
