In my experience over the last year with a centrifugal juicer I have found that if you have the leafy greens wet before they go in it helps a lot to up the yield of juice. In addition to this, you want to pack them in there as tightly as possible which will prevent them from getting pulled into the pulp extractor deal with the slightest touch from the blade.

I found that the Jack Lalanne power juicers were excellent centrifugal juicers to start with, and mine ran about $100 at Fry's. You can also pick them up at Wal-Mart, and Costco for between $80 and $100.

This is the one I have except its white. It is easy to assemble, use, and clean, and all the parts are dishwasher safe, although I have rarely needed or wanted to run in through the dishwasher.

I hope this helps you out and best of luck with your juicing!

Edit: I think as tightly as possible is an overstatement as I have jammed the plunger in there before, but about a quarter to a third of a bunch of spinach will go down at once in my juicer, depending on the size of the leaves. Whats important is to not just have the leafy stuff tossed loosely in there or it will all get pulled through looking slightly beat up.