Lapping compound helps, but if you are working on bikes at all you really should have and use a hand held impact driver. With this tool a couple of taps with a ball peen hammer will usually loosen any screw without messing up the head. You can get a cheap one with bits for less than $15, a good one for less than $50.
Here's the Sears one for $25.
http://www.sears.com/shc/s/p_10153_12605_00947641000P?keyword=impact+driver
Once you use one you will kick yourself for not getting one years ago. They work!
"I might just have to use both at the same time :)"
That would do it!