Anyone have any advice, other than further expand the clothing industry to, ah, produce *everything*?
Increase fortress wealth quickly (I usually train up a couple of engravers) and attract sieges. Destroy goblins, mass unforbid clothes, dwarves rejoice.
Or, if you have a healthy meat industry, you should be getting a good amount of hair in your refuse piles, plus you probably have some shearable animals. Spin to yarn, weave into cloth, make pants. Not as quick but easier than a plant fibre industry (if in limited quantities) if you embark with breeding animals like I do.
Don't forget that you can train your leatherworkers by making leather clothes in addition to armor -- for example, shoes, trousers, etc. Leather is a pain in the ass really but I tend to ask the caravan to bring tons, and training up on clothes rather than armor will yield a more usable product -- save the armor-making for once they know the difference between a shoe and a hat.
Plus, I find that the elves, humans, and (by default) dwarves all bring significant amounts of cloth and silk with them so just start knocking stuff out as soon as you can. The nekkid dwarves don't seem to mind what my trainee clothiers churn out.
Lastly, regarding discarding old clothing -- my dwarves seem to do it as long as there's plenty to choose from but some of them take longer than others. Some seem to wait (or are busy enough, I guess) until XX worn, some will swap stuff at X worn. If it's only X worn then they tend to put it back in the stockpiles, it's only at XX worn than they actually put them in the refuse pile. I find they are quicker to find new possessions when idle, it doesn't seem to be a high priority task (good!). Putting clothing stores near your meeting halls might be a good plan for efficiency's sake.
If you're truly desperate, drown your children, nobles and/or fish dissectors and let the survivors fight over their socks.