Tape -- When hammering a nail into a wall, avoid marking or cracking the paint by covering the spot first with 2 pieces of adhesive tape. The tape will lift off without a trace.
Paper-Towel Buffer -- Prevent scuffing when pulling out a nail from a wall by putting a folded piece of cloth or paper towel beneath the hammerhead.
Sponge -- Electric drills can shower plaster dust all over, and cleaning it up is difficult. When drilling into a wall, hold a damp sponge beneath the drill. This will catch most of the spatter and make cleanup easier.
Screwdriver -- To loosen tightly set screws, place the tip of a screwdriver into the head of a screw, and pound a few times with a hammer. The vibrations will knock the screw loose enough for easy extraction.