I've been freezing my kale in portion sized plastic take-away-food containers.
The steps I follow are: pick kale, wash to remove bugs, blanch in saucepan of boiling water for 1 minute, let cool slightly before squeezing out excess water with my hands, chop roughly with large sharp knife, pack firmly into containers and freeze.
I've found that freezing smaller portions works better for us, e.g. one container is enough for one family meal of pasta as opposed to opening a big bag of frozen kale and having to chip off a portion for the meal or defrosting the whole bag and feeling like I need to use up the whole lot that week.
In related terms, I have been pondering what greens-based meals I could cook and freeze to use up gluts of kale and silverbeet. I'm sure there are some delicious European peasant dishes out in the internet that make use of the excess. Curious to know if others have any freezable greens recipes they've tried?