It is not necessary to actually wash a sleeping bag or pad after each trip, but you might want to do it for the sake of sanitation and odor control if it has been used in a contaminated environment like around skunks or rodents. If you camp with dogs, you'll certainly want to wash it between trips. The best way to clean any outdoor gear is with soap and water - mild dishwasher soap. Make sure to thoroughly rinse with water to remove all traces of soap. The best way to dry a sleeping pad is in the sun, or if that's not an option then use your clothes dryer on low heat for 20 minutes followed by 20 minutes outside in the sun and over night air drying. Doing this process will get rid of any bacteria from mold.