My cavalier answer is lick my plate clean.

But more to the point, if we don't have water we have a problem. Water is always a consideration for us, selecting a good spot long enough to cook a meal that needs clean up.

Perfect world/planning: 3 tub method.
Usual multi-day trip: Dr. Bonners, boiled water, rag.
In a pinch: sand scrub and swirl some water around with fingers to clean out.

To my folly perhaps, I rely a lot on sunlight to sanitize my cook/drink wares between eating. I've always believed and not suffered for it yet, that if I modestly clean a cup, plate, bowl or pot and let the suns UV rays on it for a bit, I'll be good to go. I try to be as neat as possible with my cooking/eating gear anyway so maybe its just hoping it works.