Try this, from the PowerJuicer recipe book. I thought it would be pretty vile, but it was very refreshing:

A cup of parsley
A green apple
4 broccoli florets
3 celery stalks
1/2 lemon (rind on)
(I also added a little ball of spinach)

("Grind" them in the order listed)

The PowerJuicer recipe book has tons of vegetable-only, and mostly-vegetable juices. Lots of them blend the green vegetables with the brightly colored ones, with a cube of fresh ginger thrown in.