Silence squeaks. Press on the offending board, then drill a 1/16-inch pilot hole at an angle through its face until you hit the subfloor. To prevent splitting, be sure to position the bit at the center of the board and at least 3 inches from either end. Continue to hold the board as you hammer a 6d finish nail into the hole. Recess the head using a nailset, and fill the divot with color-matched wood putty.
Hide scratches. Touch them up as they occur using a stain-filled marker, sold at hardware stores.
Make stains disappear. If moisture has discolored the wood, sand off the finish and scrub on a solution of hot water and oxalic acid powder, sold in hardware stores as wood bleach. Rinse three times with water. Once the stain is gone—it may take a few treatments—make sure the wood is dry before applying a new finish.
Spot refinishing. Tape around the damaged board and sand with 80-grit paper until imperfections disappear. Sand again with 120-grit, then brush on two coats of polyurethane, allowing drying time between layers.
Fill open joints. When big gaps open up between wide planks, stuff them with manila rope or no-tar oakum, a jute fiber used by plumbers.