A while back I wrote about putting some of those little Command hooks by the front door for sunglasses (at the bottom of this post). They were fine, but then I found these at Storables: Forma Suction Razor Cups! I bought two at first for our toothbrushes, but after using them for a couple days I went back and got four more!
They're designed to hold a razor and/or toothbrush, but they're perfect by the front door for sunglasses. (I might line those with some felt, even.)
I put three there by the door, and three in the bathroom for toothbrushes and toothpaste:
So far so good on the suction cups staying put while traveling, too. We have another Forma suction thing: this clock, and it hasn't come off once after a few months on the wall over the kitchen counter.