Leafy greens are notoriously difficult to extract juice from, simply for the fact that they don't have a lot of juice to begin with. In general, however, most experts recommend using a masticating juicer for leafy greens such as spinach and kale since they tend to be the most effective at extracting their juice.
There are two basic styles of juicers: centrifugal juicers and masticating juicers. Centrifugal juicers work by grinding up the fruits or vegetables, then spinning them at a high rate of speed to essentially "throw" the juice out of the pulp. While this works well with dense, heavy produce, it doesn't work well at all with leafy greens. The greens themselves are too light weight for the centrifugal juicer to be able to extract their juice efficiently. 
Instead, the juice needs to literally be squeezed out of them. This is where a masticating juicer comes in. These juicers are designed to grind or crush fruits and vegetables, extracting the juice from the pulp and separating the two into different containers.  The pulp that comes out of masticating juicers tends to be far drier than that that comes out of centrifugal juicers, simply for the fact that masticating juicers are better at extracting juice from the pulp. This is particularly true with light weight plants such as leafy greens.
The only downside to these juicers is that they tend to be far more expensive than centrifugal juicers. However, if you want to get in on the green juice craze or just want to be able to extract juice from leafy greens for any other reason, you really need to consider investing in one of these juicers.
This video shows a comparison of how much juice you can get out of a variety of different fruits and vegetables with a masticating juicer versus a centrifugal juicer. If you fast forward to about the 13 minute mark you can see a side-by-side comparison between the two machines with kale. The difference in the amount of juice that is produced is significant.