Keeping a tyre pressure gauge in the car is a great tip on its own. If you keep it somewhere handy, you'll remember to check the tyre pressure regularly. Lifehacker reader justagigilo85 suggests keeping it in the boot of your car so that you remember to check the pressure on your spare every time you check the other tyres.
Photo by Pierre Bastien.
If you can, I'd almost suggest strapping it to one side of the boot with a little Velcro, somewhere you'll actually see it. Personally, I keep a container in my trunk with jumper cables, spare light bulbs, an emergency and first aid kit, and a few other useful things -- it's the perfect place for a tyre pressure gauge as well.
As long as it's in the boot, you'll remember to check the pressure on the spare too -- a tyre that often goes unnoticed until you need it, and when you do need it, you'll need it to be properly inflated.