HIde it near a neighbor's front porch. If found, they will break into the wrong place.
I have to second the "at work, with neighbor" etc.
Another alternative is to have one lock that is electronic, requiring a key code to enter. If you lock yourself out, just go to that door to re-enter.
My favorite? Hide a key to a non-obvious locked area. If found, the key is useless unless you know which lock it fits. Put the house key in the locked area.
Keep in mind that a key does not have to look like a key. You can trim the head to many decorative shapes, paint it, and if surrounded by similar shapes it will not be noticed.
Daniel