13thmonkey :
penetrating oil (wd40 perhaps), on a cotton bud, dabbed on the edge of the screw, wait for a few hours. Tiniest amount.
+1 and tap the screw lightly a dozen or so times after you get the penetrating oil on the screw. The vibrations draw the penetrating oil where it's needed. I've also done this with contact cleaner in a pinch, never with WD40 although its seems that would work too.
And also make sure the screwdriver your are using exactly fits the screw so you don't tear up the screw head.