What should I do with old uniforms (woodland, DCU, UCP ACUs)?

Burn em or shred em. Only way to keep them off posers. Or you could sell on eBay for a couple of bucks but then there is the poser thing again.
Best is a base thrift shop. At least that is restricted access for the most part.

Taking a suggestion from Ranger Rick's Ranger Digest III, make a laundry or utility bag. I have even used them as shoe bags. Cut off the pant legs at the crouch and select the length you need. Turn the leg inside out and sew up the wide, cut-off portion. Then you turn it right side out and use the draw string end for closure. Your bag is complete! Anyone have ideas for recycling other parts of the uniform?

I've keep one set of each for keep sake. The rest went to the trash after worn out. The idea of giving the to the medics or other sections for training (i.e OPFOR) is a good idea too.