First place: Turn me on again
After taking all appropriate steps to ensure you are working safely with circuits that are de-energized, and when work has been completed and circuits safely re-energized, write a Post-It note reminder or record a voice memo to yourself to be certain that equipment such as refrigerators and freezers are turned back on at the completion of your work.
On the level
The inventor of the magnetic level might not have imagined that a magnet would be handy for picking up metal shavings like the ones we always have to remove from the top surface of a panel or junction box. By wrapping the magnet with a thin rag or using a piece of tape with the sticky side out on the bottom of the level before you start the cleanup, you can capture and dispose of those little bits of metal very easily.
Oldie but goodie: Staying in its place
This idea is for when I measure undercabinet lights to the center of the cabinet, keeping it straight to the wall, then holding the fixture to mount it. (This is all done while leaning over the base cabinet.) I put two pieces of double-sided tape on the back of the fixture, which holds the fixture in place so the mounting screws can be installed and wired without moving from my marks.