Unless you have a masticating juicer you're yield on leafy greens will not be optimal.
That is not to say that you can't get all you can and need out of them.
For spinach, I usually roll the leaves up and add them with a much harder freggie, maybe an apple or a carrot. If not I will just roll them as tightly together as possible(think cuban cigar) and feed the rolled leaves up as slowly as possible.
As far as kale, I found that if I remove the leaves from the stalk from top to bottom in one piece, and roll the leaves on the stalk (again think cuban), I get more juice.
I have a centrifugal juicer too, so I had to come up with a way to get all I could from my greens.
Hope this helps some, it's the best way that I have found to get the juice out.
| current weight: 279.0