the screws have a little magnetic energy in them... go to a hardware store and get a little magnetizer cube for a screwdriver, or bring your screwdriver to the store and just rub your phillips in there for a few seconds... also, you can get a smaller phillips... iPhones use #00 size phillips heads, which even without a magnetic force; the screw sits nicely in the head and doesnt fall out...
It takes practice, but you'll get it... with or without gravity!