A related idea: if the front and back doors of your house use different keys, and you carry both, consider changing the locks so the same key fits both.

I've done this at two houses and I find it makes life easier. I believe it is marginally more secure, too. I gather the disadvantage is that if you lose a key, you have to change both locks rather than one; but in practice if I lose one key I'll probably lose the other at the same time. Ditto if you lend a key to a friend who you later decide not to trust.