Smoothies are a great way to pack in lots of nutrients that you might not normally get enough of. For example, flaxseeds are great for us, but who eats 'em regularly? Not even this health nut. Uber-healthy greens? Even this super salad eater could use some more. Berries? You get the point.
There are many variations, but in general, I try to have at least one thing from the first three categories and use the other two to make it taste good, blend well, and use up the fruit and veg in the house.
Greens- If you use spinach or romaine lettuce, you don't need to mask the taste. If you use stronger tasting greens like kale or chard, which is more nutrient dense, you might need a banana mixed in. This masks the taste of the greens. Add as many greens as you can "to taste." Start with a little and over time, you'll find that you can toss in a ton and it still tastes fabulous!
Berries- I always have lots of frozen bags of berries. . .wild blueberries, strawberries, raspberries, blackberries. Use a variety.
Seeds and/or Nuts- Flax seeds are a great source of omega 3 fatty acids, but they need to be ground for better absorption, so make sure you grind them yourself or buy them pre-ground. (They help to prevent breast cancer too!). Walnuts are also a good source of omega 3's and they are good for your brain. I always toss in some other seeds and/or nuts for variety and diversity of nutrients. If you eat nuts throughout the day already, you may not need the nuts, but do add the seeds.
Other Fruit or Veg- I always have frozen organic fruit that I can use (peaches, mango, etc). I also use any fruit (and some veggies) that are about to expire. Nothing goes to waste. If it's not being eaten directly, I make sure someone gets the benefits of those pytochemicals! I always have an apple core in there since my girls take apple slices to school every day. Carrots taste good. Cucumbers are refreshing. Any melon.
A liquid source- I use either non-dairy milk or juice, but just enough to make it blend well. You could also use ice cubes. I most often use juice from a fruit that I do not normally get otherwise. . . pomegranate, cranberry, acai, black cherry, etc. (Note: Read the ingredients. Many juices are actually just a blend with apple juice as the first ingredient or they actually have very little of the fruit that they are trying to tout). I like Lakewood. Their cherry juice is only pressed organic dark cherries, for example. DO NOT use dairy milk. Milk blocks the phytonutrient absorption. I.e. you will cancel out the benefits of some of your smoothie ingredients if you use dairy milk. What would be the point then? http://nutritionfacts.org/videos/nutrient-blocking-effects-of-dairy/