1. Make sure you have the right screwdriver. Drivers and screws can vary, so the best way is to check it yourself.
2. Put a dab of superglue on the tip of the screwdriver, hold it for a few seconds to let it bond, then use a lot of downward pressure as you rotate.
3. If that fails, use a dremel to cut a notch in to the screw, then use a flathead screwdriver to remove it. Cover the rest of the machine with plastic first so the metal shavings don't touch any electrical components.

Also see this post about stripped screw technique.