Long ago, I sold carpet. The biggest problems I encountered was people using too much detergent when the steam clean. It can leave a residue that will attract dirt.
As far as stains go, I always suggested treating the stain with a spot remover, putting a wad of paper towels over the stain and putting some weight on the towels. The stain will wick into the paper towels. Three kids later and this is still the best technique I've used--outside of professional cleaning.