I made smoothies every morning for a couple of years.
I alternated juices so they didn't get "stale," but prefered a blend rather than a single fruit juice. A couple of big-heaping tablespoons of raw wheat germ, one raw egg, a dollop of milk...blend like crazy.
Since raw eggs have been linked to samonella in recent years, I decided to forego the raw egg...and it took quite a bit away from the smoothie.
My favorite juice combo was POG, either Pineapple, Orange, Guava, or Passion Fruit, Orange, Guave. Both are good, with greatly different flavors.
I can't stand yogurt myself, but it makes a good smoothie ingredient for those who like the nasty soured-milk tasting stuff.