A couple of tin cans to put behind the door or a string of chrstmas jingy-bells to hang on the door knob, and a hardwood wedge (two if there is a "porch"). You can actually get small motion detectors now, they aren't super expensive, but this option is cheaper.

When I've known I was going someplace interesting, I've thought about bringing an old leg hold trap with an 60 pound spring. Haven't yet, but it will slow the first guy though the door down if I ever do. <img src="/images/graemlins/smile.gif" alt="" />