Now, I always though you cleaned everything after one wear, but I've found it's more commonplace not to do that. So, how many do you usually wear trousers before washing them?
If we are speaking of wool, believe that dry cleaning should be done only as often as dictated by degree of dirt, odor and wrinkles. Believe that dry cleaning ultimately damages the fabric. Therefore, I brush my woolens after every (or almost ever) wearing, and try to air them out outside when the weather is nice.