Leo Babauta in Ultralight:
Be willing to wash your clothes by hand every 2-3 days to prevent having to bring extras. You can wash less often if you're willing to wear your clothes a few times before you wash them. For example, I can wear a shirt for at least three days before it needs washing (unless I sweat heavily in it), and pants can go 5-7 days (depending on dirt and sweat levels of whatever activity I'm doing). Underwear is something I only wear once.
I recommend handwashing to every traveller. In most packs, clothes take up the most unnecessary space and weight, so reducing extras has tremendous benefits. Synthetic clothes are vastly superior in this regard, because they can be washed easily by hand, and they will dry in a few hours. Jeans, cotton pants and pullovers are the least suited for travelling. To speed up drying, I learned this trick from Leo: Wring out a piece of clothing and then roll it up with a towel underneath, so the towel ends up on the outside. Then, wring the towel roll as much as possible, and the item might even be ready to wear again.
Also, fewer clothes mean I don't have to decide what to wear in the morning.