ummm... let's see.
1. Fruit is your friend. It's the staple of all smoothies. Frozen fruit will often lack the flavor kick you need though.
2. Supplement the fruit with additional flavor. This depends on your tastes. If you don't mind sugar, add in some sort of syrup. You can also change it up with fruit juices, juice concentrate, sorbet, or ice cream.
3. Smoothies are generally better when you can identify what's in it (strawberry, mango. Pineapple, peach.) rather than a vague fruity sweetness.
4. Dairy products will whip. Along with the taste they add, you'll add more air into your smoothie, which will often make it seem thicker, but this is an illusion. If you substitute juice with milk, keeping all other quantities the same, you will have "more" smoothie. Also, bananas will give a smoother/thicker taste to your smoothie (although it'll taste terrible, along with everything else you decide to put banana in)
PS: if at a boosterjuice or jambajuice, ask for no yogurt. It'll give you more quality smoothie, and you won't look cheap doing it (some people don't like yogurt, or are allergic.) If you don't care about being frugal, ask for no ice as well. This will often cause problems at the blender though, and your smoothie will take longer.
5. If you're adding a supplement like whey protien or any other distasteful ingredient, I've found that a sweet, rather than tangy, taste will better cover the flavor of the particular supplement. If you're looking to make a smoothie where you want to taste the subtleties of all the fruit, don't put in a supplement- unless it's relatively tasteless on it's own, it'll taint everything in the drink. Use strong, sweet flavors for supplement smoothies.
As for recipies, play around and mix different fruit combinations. Everyone's tastes are different, so experiment until you find one you like best.
