A note about Greek yogurt: Greek yogurt is thick and tangy and thus lends a smoothie more substance and flavor. I recommend Trader Joe's brand (it's the cheapest), but FAGE is also wonderful. If you want the smoothie to be sweeter and less thick, try substituting vanilla yogurt.
1/2 cup blueberries
1/2 cup strawberries
1 ripe peach
2 key limes or 1 regular lime
A dash of orange juice
Plain nonfat Greek yogurt (about 1/2 cup)
Ice cubes ("to taste"-I used about 8)
Take the blueberries, strawberries, and peach and put them in a blender. Add the lime juice, the orange juice, and the Greek yogurt. Blend until smooth. Add the ice (use less if you're substituting frozen fruits for fresh ones) and blend until smooth. Serve with a garnish of lime.