I wash it, chop it, and cook it down. Then I put it in freezer-safe storage containers and pop 'em in the chest freezer. If I'm feeling particularly industrious, I'll buy a giant mess of onions and garlic and cook those beforehand to add to the greens. One onion and two or three cloves of garlic for every two bunches of greens.

I do the same thing with the big bags of baby kale from Costco. Two or three onions per bag for that. When I spend a day cooking greens I usually devote four hours or so to it and I wind up with probably twenty cups of greens.

Two big bunches of greens = one sink full = one two-cup container (with added onions)