Well, you're in luck! The godlessness of the town of San Francisco is offset by its environmental zeal -- so if you've got a tree you need to get rid of, drag it to the curb on your regular recycling day (which is one day later than usual this week) and the city'll take care of it for you. You can also drop off the tree at the recycling center by Kezar Stadium (Jan. 1-15 only, 9-4 M-Sat, 12-4 Sun), or outside Golden Gate Disposal at 900 7th Street any time. You can call your garbage/recycling service to pick up the tree at your home too (as one of your two annual bulky item pickups, we think.) Take off all tinsel, stands, lights, wires, and other decorations (and those trees with fake snow on them can't be recycled at all).
In San Francisco, the recycled trees are sent through a wood chipper and used for mulch, placed under park benches, and used to control weeds by the freeway. You can't compost the Christmas tree in your own backyard (the tree is too acidic) -- and recycling Christmas trees as opposed to just tossing it out helps cut down on Pitch Canker disease, which is affecting 19 California counties and devastating native growth trees.