DC Department of Public Works will pick up (real) Christmas trees and holiday wreaths between December 28 and January 10 for composting. Remove ornaments, tinsel and other decorations and put the tree(s) and greenery where your trash and recycling are collected. Trees and greenery collected after January 10 will be collected with the trash as space permits in the truck. Here's a link to DPW's brochure on trash and leaf collection. And, if the leaf raking crew hasn't made it to your block yet, it's time to give DPW a call at 311.
Collect holiday (and other occasion) cards (in your neighborhood and at work) and send them to St. Jude's Ranch for Children. See details for this program here.
Meanwhile, instead of throwing out unwanted but usable decorations, tinsel, lights, gifts you really don't want, ugly holiday sweaters, bows, ribbon, wrapping paper, etc, try listing it on FreeCycle DC. Happy undecking!