It's very important that when you travel that you must pretend that you are back in the Stone Ages at times. Sometimes, this even applies to laundry.
Generally, my rule of thumb when it comes to packing is that you shouldn't be taking more than a week and a half's worth of clothing with you - and that's for the absolute, longest trips possible, even if you're going for a year. Not only is it to prevent overpacking, which is only the greatest sin of 'em all, but most people don't realize how simple it is to keep your clothes clean while you're on the road.
So occasionally you wear your jeans more than a few times, but you definitely have several options for keeping your clothes pressed and fresh. Usually the easiest and most expensive way is to ask your hostel or hotel take care of it for you. Besides knowing every option available, most places offer services where they can get it ready for you within a day or two if you're running particularly low on clean underwear. If you need it for the next day, though, be sure to ask first thing in the morning.
The second option is to find a laundromat, which often comes with the challenge of finding detergent. Luckily, some places offer it in expensive, bite-sized pieces. But if for some reason it's not offered, you might have to make a quick run to the supermarket or even substitute something like body wash for detergent. Which brings me to my next point, if you're starting to get desperate.
At the risk of sounding like a complete hippie, handwashing is definitely an under-utilized option if you're in a slightly less advanced country or if you don't have enough dirty clothes to require a full load. Besides the prerequisite sink, you generally don't even need detergent; in fact, even a body wash, bar soap or shampoo that has a nice white scent is sufficient. You won't want to go overkill on the whole thing-a teaspoon or two seems to work just fine-but after lathering it up, you'll make sure want to twist and wring out as much water as possible so it will dry faster.
Unfortunately if you're planning to do it the old-fashioned way, keep in mind that hanging and drying can take up to a day or two depending on the environment you're in - that's unless you're in the desert, in which case it will probably dry faster than it would in a commercial dryer. Depending on what kind of air circulation and humidity you have, handwashing can often rather inconveniently require some advance planning. If you plan on doing this quite a bit you might want to pick up a clothesline and stick to light, breathable fabrics. It is the cheapest and sometimes the most efficient method, though.
I mean, everyone washes their clothes somehow. Did you not see me washing my clothes in the lake?
