The straps: match the method to the material
Suede or nubuck straps want a dry suede brush and a suede eraser for marks. Brush in one direction to lift the nap, rub the eraser over stubborn spots, then brush again. Resist the urge to wet them; water on suede leaves a tidemark that is far harder to remove than the original mark.
Smooth leather straps can take a cloth wrung out in soapy water, then a proper leather conditioner once fully dry — leather that has been in sun and sweat all summer is thirsty, and conditioning now prevents the cracking that shows up next spring. Synthetic straps are the easy ones: soapy cloth, rinse cloth, dry cloth, done.
The cork rim is where sandals actually die
The sealed cork edge is the part almost everyone neglects, and it's the part that determines whether your sandals last three summers or ten. Once that sealant wears thin, the cork absorbs moisture, dries out and starts to crumble at the edges.
Check the rim in good light for hairline cracks or patches where the finish looks matt and dusty rather than smooth. If it looks dry, apply a thin coat of cork sealer with a small brush or a cotton bud, keeping it off the footbed and the leather, and let it dry fully before the next coat. One or two thin coats beat one thick one. Do this at the end of the season rather than the start and you'll pick the sandals up next year ready to wear.
