I grew up in “Lynn, Lynn, city of sin” – so one of the first things my parents impressed upon me was the necessity of closing the windows and locking the doors on our family car whenever we got out. That worked well until the day my father stopped at a store with my sister and me, and we duly locked the doors … to find that Dad had left the key in the ignition :-(
He grumbled as we walked home (only a half mile, but he’d also need to walk back with my mother’s car key) and thus taught me to always carry a spare car key in my wallet to avoid this problem – because no matter how careful you are, a second’s distraction can lock you out, perhaps much further than half a mile from home.
But as cars have become more high-tech over the years, so have their keys. Chances are that your car key now has a thick plastic head containing a programmed electronic chip that interacts with your ignition switch lock to allow the car to start, like this:
However, if your key is locked inside the car you don’t need to use the spare key to start it, only to get inside to access the other key. So as long as you have one door with an actual keyhole (my Subaru only has a keyhole on the driver’s door, since they expect that normally I will unlock the car with the remote fob) you can go to your local locksmith and have him/her make you a “naked” key without the chip head, which will fit in your wallet with minimal bulk:
Do check your manual if your car has a security system, to understand whether using a naked key will set off an alarm – and if so put a note into your wallet with the key detailing how to STOP the alarm! (e.g., some cars require you to insert a headed key into the ignition and turn it on/off X times).
To keep this key from sliding out of my wallet, I wrap it in a $20 bill (having also learned to keep emergency money for a cab or other use), and if I don’t have a zipped/velcro’d closed wallet slot I put it in a sleeve with my ATM card:
One final tip: if you also want to guard against losing your key after successfully remembering to take it with you, hide a fully functional chip-headed key somewhere inside your car (not the glove box, unless it has a good lock; duct-tape it under the carpet in the trunk or somewhere else not obvious if you forget to actually lock your car door) so that once your spare key gets you inside the car you can still drive home.