In a blender combine:

• 1/4 c plain yogurt (I actually use vanilla for mine
    but it would depend on what else you're adding)
• 1/4 c skim milk
• 1/2 banana
• A splash of vanilla
• A dash of cinnamon
• 4 ice cubes

Blend to desired consistency.
Substitute whatever fruit is in season or even use something like canned peaches.
I usually double the recipe above but this is what I remember
the original measurements to be.