I often freeze greens (like collards). After they're cooked, I just place them in a single layer on a pre-chilled sheet tray and pop them in the freezer for a few hours, until they're totally frozen. Then I wrap them in plastic wrap and/or aluminum foil and into a freezer bag. They reheat very quickly and taste not much different from fresh. Next time I might try running them through a salad spinner or drying with paper towels to remove excess moisture.
I have also frozen winter squash and sweet potatoes. After cooking, I put them into old yougurt containers and put them in the fridge overnight. The next day I stuck them in the freezer. I have also frozen caramleized onions in the same manner. I did this because, from what I understand, the quailty of a frozen food is better if the transition to frozen is as fast as possible.