Lockout protection. (I think I mentioned this one waaaay back when, but here it goes again.)
Use rubber cement to fasten a spare key for your car to an old credit card, and carry it in your wallet. If you lock your keys in the car by mistake, this will give you a second chance. Unless, of course, you lock your wallet in the car as well. (Did that, once. sigh.) If you’re a two car family, a second key will fit.
In fact, most of the time you can fit three keys. I also happen to have a spare housekey on the card. I thought about it for some time before doing this, as it means anyone who steals my wallet has access to my house. Two things decided me, however. First, I carry my wallet in a difficult to reach location – not impossible to steal, but difficult. Second, I sometimes get taken places and don’t carry my keys, but I always travel with my wallet. If I get home and nobody’s there, I can still get in.