When I joined Airbnb and came to this forum, I searched for good ideas and one of the good ideas was to buy a waterproof mattress cover. After two months, I can say it is the best $25 I have spent. And at Target the Allerese ones are currently on sale - they are zippered and washable! I was at work and a guest informed me of a spill on the mattress - I didn't get to come home to address it for 6 hours. Still nothing went through to the mattress and I was able to wash the cover completely clean. Worthwhile investment, because life happens.
Good tip! Although I wonder if I can find one the right size to fit my futon mattress (thinner than an average mattress).
I also use the zip allergy cases for my pillows that provides a protective barrier against dust, mites and germs between the pillow case and the pillow, prolonging the life of the pillows! They do vary in cost and quality between cheap light plastic which rustles and tears, and heavy duty cotton that can be washed repeatedly in hot water.