Last year my friend locked herself out of her house several times, and her car a few. I took the hint, and had extra house and car keys made. I have 2 sets of car and house keys on twine. One string goes through a belt loop of one pair of jeans, the second set through a second set. I have an extra set of car keys in my purse and in the car, a key safe stashed at my home, and a car door key attached to 2 painting boxes, in case I lock everything else in the car when I get out to paint... And with a locking fuel tank, there is a spare key in the van for it, too.
I had to be towed recently. The driver had my key ring and was working with my car. At one point he came to me (I had been sitting in his cab for several minutes) and with a grin asked me if I had a spare key, that *I* had locked the door with the keys inside! I smiled and said yes! He said, crestfallen, "You do?" He looked unhappy as he watched me pull them from my pocket. I figure he planned an extra charge to have his wife or buddy bring a slim jim from home.