"Something as simple as leaving a bed unmade during the day can remove moisture from the sheets and mattress so the mites will dehydrate and eventually die."
Leaving the bed unmade (sheets and covers folded down and back at the foot of the bed, so not entirely messy) allows any sweat to dry, and lets the bed air out. Gross, but real.
http://news.bbc.co.uk/2/hi/health/4181629.stm