In addition to the penetrating oils you might want to try an impact screwdriver. The hammer powered kind, not the air or battery powered ones. They usually have higher quality bits and can be had for $10-20. They work amazingly well for breaking screws loose.
Once loose, if the formerly exposed threads are hanging you up use lots of penetrating oil and move the screw in and out. If you take your time you can usually get them out.
Chuck
