A smoothie needs four things:
I combine by using fresh fruit (flavor), frozen fruit (flavor, frozen), frozen juice concentrate (sweet, frozen), and yogurt or whole milk (liquid). (Sometimes I use yogurt AND milk, depending on the other ingredients and the proportions.) I don't like to use ice cubes, as that waters-down the flavor.
It is worth mentioning that not all fruits are created equal in the world of smoothies. I have found that grapes and melons make pretty poor smoothies. Apples give a strange texture if they are fresh, but are a great addition when frozen. Apple and grape juice concentrates are the best sweeteners--white grape and apple will not affect the flavor much either.
I love throwing a banana into almost any mixture. They are heathy, cheap, and add nice texture to a smoothie. I use them fresh at first, then they start getting a little brown, I put them in the freezer and use them frozen.
I start with a half cup or so of plain yogurt in the bottom of the blender, then add fresh fruit, then frozen fruit, then sweetener, then pour enough milk to just barely cover everything. (Note, if you have a lot of soft or juicy fruit, use less milk!)
Some favorite combinations:
Fresh peaches, frozen strawberries
Fresh kiwi, frozen strawberries, (can add fresh or frozen banana)
Banana and strawberry--either one fresh or frozen
Fresh raspberries, frozen apple
Fresh cherries, frozen apple, optional banana
Any mixture of berries
If you want to feel inspired, go down and look at the yogurt in the grocery store--any fruit combination that they make there is bound to be even better in a smoothie!