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You can use a "corner reflector" that directs the beam back to the original target no matter where it was aimed from. Different thickness and materials cam be used for different types of lasers and frequencies. Using vapor deposition, you can get very uniform surface coatings.

A corner reflector. Now that's interesting, presumably something akin to reflectors used in some cameras and other optical equipment to see 'through the lens' when the eye isn't directly in line with it? Add another and you look at your own eye. How stupid of me not to think of that, particularly with my camara sitting right here by my computer. It would be a bummer to have a big laser foiled by a set of mirrors, or worse, turned back.

I'm still wondering about what would happen if a powerful laser hit a damaged mirror/reflector, though. I know people test lasers using well-made mirrors because the idea is to make the laser work- I'm just curious if anyone has tested a laser on a deliberately flawed mirror, since under field conditions a reflector might get damaged and be only partially reflective. If it loses too much of its reflective surface area, would it absorb the heat rather than reflect it, and possibly be damaged even more?

Why use something as complicated as a mirror? A better defense would be plain old water vapor, i.e., a cloud. Lasers do have problems penatrating cloud cover. If you used smoke particles mixed with very finely misted water vapor, the laser would have trouble breaking through while maintaining coherency.