I have a gas powered warmer below my oven. You only see the switch for it when you open the warmer and it located below the oven near the floor, so it is very innocuous. Whenever I use it I forget about it and inevitably there is some food that gets left in there and "warmed" for 2 or 3 days (or more) before I notice somehow and turn the thing off. I only use it when I am entertaining, so the environment it gets used in is distracting to begin with.
I need some easy way to remind myself to turn off the warmer after the party is over.