A while ago I remember Cooks Illustrated did an article about perfect smoothies. Unfortunately for me, their perfect recipe was made with milk, and my insides can only tolerate so much dairy. I like to use yogurt in my smoothies, so there is some protein, but if I add milk to that, I have a miserable day.
Sorry. More information than you needed, I know. But the point is that ever since I started making smoothies for breakfast four or five summers ago, I have been trying to figure out what liquid to add so that they don't have the consistency of oatmeal.
Sure, you can use rice milk or soy milk, but those always make the flavor just a little odd. Though if you don't mind the slightly odd flavor, and the slightly brownish tint (which I think tends to ruin the bright freshness of the smoothie), then soy milk is your answer, given all its nutritious goodness. Rice milk doesn't seem to add much, I've found, except a little milki-that-is-not-milk-ness.
But apple juice (and probably white grape, though I've not tried that) keeps the fruit emphasis, and you don't use enough to mask the flavor of the real fruit in there. Also the juice adds a little sweetness so you don't need to add sugar.
Also, using frozen berries (that you buy frozen or freeze yourself) means you don't need ice, which can thin out the whole thing.
Here is a decent mixture. I often double it.
1-2 cups frozen berries (blueberries or mixed)
1/2 cup apple juice
3/4 - 1 cup plain yogurt
Then blend the hell out of it. Feel free to play with the proportions to suit your preferences. Play with seasonal fruits. Play with frozen fruits (frozen mango chunks were disappointing un-mango-ish). When it's peach season around here, then peach is the answer.
Because I could not decide between Sade and Rob Thomas, there is no musical accompaniment. Sing to yourself.