I'm heading to Mexico in a few days with a fresh pair of sandals in my pack. I didn't buy new ones, I just cleaned up the old ones very well with some Nikwax sandal wash.
Nikwax is known for their line of waterproofing products, which work very well at turning a pair of hiking boots into water-repellent boots. They also make some spray-on waterproofers that work for jackets and the like.
This sandal wash is meant to combat an entirely different problem though: funky footwear. This stuff kills bacteria, deodorizes, and gets rid of everything that's been stuck to your sandals after months in the outdoors. In my tests it was easy to use (there's a sponge applicator brush) and delivered as promised. One $7 package should last you for years. Besides the obvious use on sandals, you could also use this for Crocs and other rubber or plastic footwear.
Get yours at REI, Backcountry.com, or Sierra Trading Post.