Where is it possible to bring old clothes, shoes, sheets, towels...etc in Kunming, for recycling or for associations helping poor people?

If they are in good condition, in your xiaoqu, split them between the cleaning ladies, gardeners, security guards, construction workers.

some residential areas have old men/women with carts loitering around the streets. They'll BUY your old junk and resell it at swap meets, etc.

as people on here have said, spit them up into clean bags/piles and leave them next to the qiaoqu gate or bins. anything even remotely useful or of any value will be gone in seconds. if you have anything really good you can give it to the 'guards' and they'll get it to the right place or person.

There is a big outdoor market that starts every evening around 7pm on Panlong River. The location of the market is on the river between Baiyun Lu and the railroad tracks (its very close to the railroad tracks on Panjiang Dong/Xi lu). They sell everything there and would be happy to have your old clothes donated to the market peddlers. I showed up with a big bag of clothes and walked away with about 100RMB. Not my intention to sell anything, but thats how it ended up... Definitely worth a look.

If you decide to leave them unattended around your xiaoqu, there's a good chance they'll get snatched up before they even hit the floor, usually by the bored mother in laws who live with their son/daughter and are looking to make some quick kuais for their mahjong sessions.

Nothing will be wasted, it all gets recycled informally, as said above.
We give our stuff to the gardeners and guards. We get better service as a side benefit.