Here's how I make smoothies. I think they taste really good. You just blend yogurt, milk, ice cream, and frozen fruit all in equal measure. I usually don't use ice cream and it still tastes good. You can also add sweetened condensed milk if you want it thicker.
If you don't use ice cream and you use plain yogurt you may need to add a little sugar. If you use flavored yogurt it should already taste good.