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In addition to using a penetrating oil, tapping on a bolt can work wonders. After applying the penetrant, use a hammer (and a drift punch if necessary,) and beat on the head of the screw as hard as you dare. If you expect that you may break the head off the bolt anyway, try using an impact wrench. (Start with low pressure and massage it for a while.) Vibration will help a lot.
Also, extreme heat does work. A map gas torch may do the job. Sometimes acetylene will be necessary. Alternating a few times from heating the bolt only, to heating the bolt's seat only can help loosen things. If it's a hardened bolt, don't get it too hot.
One thing I cannot overstate is how important vibration and shock is for loosening screws. I've used enormous impact wrenches to remove hub nuts without success. I've massaged the same part for twenty minutes with a smaller impact wrench until it finally just unscrewed easily. I feel like a dumb monkey beating quarter-inch screws for several minutes with a three-pound sledge, but it just freakin' works!
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