1. Have a tube of all-purpose strong glue on hand for minor repairs. Also keep Teflon tape, electrical tape and sturdy double-sided tape on hand for piping, wiring and general repairs.
2. If you are working with wood and mis-judge your sides and cut it too short, cut the piece you have made in half and then make up the mismeasurement with a different piece of wood. If you wish, you can use a different shade of wood to create a decorative accent, or just stain the wood all one color so that few people will ever see your mistake.
3. To prevent paint from spilling all over the edge of the top of the paint can, punch holes all around the cover rim. When you swipe the brush against the rim, any excess paint that bubbles onto the rim will automatically drip back into the can.
4. When painting the house, wrap doorknobs with plastic bags and cover plate switches with masking tape to keep them free of paint splatters when repainting walls and doors. If you remove them, put them in separate plastic bags with their screws and any other fittings in order to avoid confusion. If you have a lot of these fixtures, consider writing the location where the items came from on an envelope and putting the plastic bag inside the envelope.
5. Old plastic shower curtains make handy protective drop cloths while working/painting to protect the floor from splutters and drips. After all, you are only going to throw them away anyway, so spray them with Lysol or another disinfectant and hang them for a day or so to be sure they are completely dry. Then fold them away and store them until your next paint job (which will usually be about every 3 years depending on the quality of the paint you use.