Gas On/Gas Off

I’m 68 and have been worried about my memory and thinking for a couple of years. My wife and I started noticing that I left the gas on after cooking dinner, which of course is really dangerous. And I do all the cooking at our house, because my wife is unwell. I never had any trouble remembering to turn it off before, but I started leaving it on more often. So I made a little piece of card, which on one side says ‘Gas On’ and on the other says ‘Gas Off’. So now I’ve got in the habit of putting it in front of the gas stove when the gas is on, and then later I can check to see that I’ve flipped it over to the ‘Gas Off’ side. But I’m not sure how effective it is, because I still have to remember to look at it! I’m not sure how I can improve it. Any suggestions?