This isn't exactly locksmith related but it's probably helpful for most people at one time or another... A rusted screw can turn a quick and easy job into quite a headache when the head wears away.
My hint is to firmly tap the screwdriver with a hammer before attempting to turn it at all. This pressure will 'break' the corrosion bond and allow the screw to turn a bit easier.
If the screw won't turn after that, try tapping the screwdriver with the hammer as you slowly turn the screwdriver.
Of course... you will need to make sure the correct size screwdriver is being used in the first place!