When I was trying out a raw food diet, I used to do green smoothies for snacks. I'm a little picky about textures and colors, though, so my green smoothies were never really green :-) (in color, that is). I also didn't have the courage to stuff a bunch of greens in there - mine were dominated by fruit.
I think my usual recipe was something like this: 1 cup packed spinach leaves, 1 cup frozen fruit, juice of 1 orange, a touch of agave (if I needed the sweetness) and water as needed. I may have thrown in a tablespoon of ground flax in there as well, I don't recall.
They were very delicious, as the spinach gave the smoothie a very creamy texture but didn't dominate the taste. I tried smoothies with other greens (parsley, lettuce, celery) and just found them too dominating. Spinach always worked well for me.
I haven't had a smoothie in more than a year because I'm in Israel for the time being and they don't have fresh spinach here. I kid you not. Fresh local grown produce is very widely available here as Israelis are big veggie and fruit people but because it's focused on local stuff, I guess that spinach doesn't grown well in a Mediterranean climate

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Tam