I keep a spreadsheet of lists for backpacks, BOBs, and any time we go travelling. If I check off everything on the list, I know I'm covered. It also has things to do such as turning down the water heater, turning off the water, etc.

It's pretty easy, I just carry a 3 x 5 card with me, and when I forget something, write it down. When I get back home, it goes on the list.