Sure there is, you just have to make conditions unfavorable to bacterial/fungal growth.
Either outright kill them with antibiotics, or perhaps alter the pH of their environment drastically, either towards alkaline(baking soda), or acid(vinegar).
Baking soda works very, very well in shoes. Probably a little more hit or miss with sandals, you would have to apply it every day or something.
If you can get some plastic sandals, it'd be super easy to wash them in some soapy, bleachy water every week..
Also remember to SCRUB your feet when you take a shower. A medium-hard bristled brush is a must in the shower, IMO.
Sometime after you've been in the shower for about 20 minutes, take and run your fingernails forcefully across your feet in different places. You will probably be horrified at the ammount of dead skin you come up with under them. That's some of what the baddies are feeding on.