I buy the big back of power greens from Costco, You can easily buy a bunch of kale, chard and spinach at any grocery store.
I slowly keep adding more and more greens to the blender, I add a little but of juice or water just to help it combine. You really don't need much.
Once all blended I put them in ice cub trays and freeze.
Once they are frozen I take them out of the trays and put them in a gallon bag. I add 3-5 cubes per smoothie, just depends how grassy you are ok with.
By doing it like this I have less greens go bad on me and I seem to add more greens to the smoothie than I normally would.