I recently got two 12" subs in my trunk, and ever since I installed them, I've been having a hell of a time getting my mirror to stay put. Every time the kick would hit it starts pointing at the floor. I didn't want to glue the mirror in place for resale purposes, so I ended up buying some black silicone gasket sealant and putting it on the ball-joint of my mirror. It's soft enough to allow me to adjust the mirror and to reduce vibrations from the bass, but solid enough to keep the mirror in place. Took about 2 min to apply, and I was done. The best part is that you can't even see it because it blends in with the plastic.

Subs - $600
Gasket Sealant - $6
Being able to drive without one hand on the rearview mirror - Priceless