At Trinidad Gonne-smything School we were told to hollow grind conventional screwdrivers (or file them if you don't have a grinder) such that it perfectly fits the screw you're working on. If you're working on an expensive gun, you'd better make sure that screwdriver fits. Like Old Soldier said, push straight down to avoid slippage. We were also told (Gunsmithing Repair 1) to buy used screwdrivers at pawn shops or garage sales for the specific purpose of specializing them for guns.
All comes to worse, put a few drops of Kroil (that's the penetrant we used at Trinidad) down the screw (and give it time to work its magic), mount the screwdriver in a drill press and the workpiece in a padded vise. Lower the chuck until the screwdriver falls into the slot. Lock the chuck in place and then manually rotate the chuck.
If it's still stubborn, you may need to apply heat from a propane torch.