Trying to Break Loose a Bolt?

Quick question guys. I'm changing the starter on my 91 Pony Escort and I can't break loose one of the bolts. I tried spraying WD40 on it and it didn't do a thing. What is good to use to break loose bolts? I've been driving the 00 GT everyday and putting too many miles on it. lol Thanks for the help.
Todd
 
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