The easiest way to keep a screw on a screwdriver when attempting to get the screw in the hole is to use a magnetic bit on the screwdriver. But this isn't always an option, and even when it is, some screws are not magnetic. In those cases, a trick that often works for me is to use a separate substance that will help keep the screw in the screwdriver. Some substances to consider for this are: vaseline, lip balm, glue (if it is thick enough), epoxy (just use one of the tubes so it doesn't harden), or auto body filler (Bondo). Just put this in the screw head and then insert the screwdriver, the material should help keep the screw in place. One the screw is inserted, remove the material from the screw head and the screwdriver.