Green smoothies are becoming such a rage, and they are a fad that I applaud (for the most part).
Green smoothies are drinks made with green veggies. The biggest advantage to using green smoothies is that most people don't eat enough green veggies, so beginning a new routine with green smoothies is usually a great step in the right direction. Green veggies are some of the most health promoting foods around.
Ideally you would use a blender powerful enough that you can put in the whole food and it will all come out liquefied. This way you get the whole goodness of whole food. I strongly suggest that you do not use juicers, since they throw out the pulp and you no longer have a whole food.
Digestion is generally a problem associated with most disease states. Pulverizing food in a blender can help with this for some people because the food is already broken down beyond what chewing would do. Thus the food is easier to digest. Most of us do not chew our food as well as we should.
Though having said that I would caution that it is still best to chew all food before swallowing. Chewing does much more than just break food into smaller parts. Chewing actually starts the digestive process using enzymes in the mouth. Chewing also triggers the stomach to recognize that food is coming and to start preparing the acids and enzymes it needs for digestion. Also chewing allows our taste buds to taste the food. Taste is also an important part of eating. It not only can tell us when something we put in our mouths is rotten, but can also help us to know when we have had enough food. Many nutrients actually enter the blood stream directly from the mouth as well.
I would encourage you to at least minimally chew each mouth full of smoothie before swallowing. You will get more goodness from it.
Breaking the veggies down also allows for the release of many of the phytonutrients and antioxidants they contain without the digestive system having to work so hard. With these nutrients more easily available, the body can start its rebuilding and repair projects quicker and easier.
Another important thing made more useful and available is the soluble fiber from veggies. This fiber is a major aid to digestion and nutrient absorption. As well as helping with the clean up through bowel activity. If you use a juicer, this is what you are throwing out with the pulp (Don't do it!)
Caution should be used in choosing green smoothie recipes. Some call for added sugars (believe it or not) which greatly reduces the health benefits of the veggies. Don't get caught in that trap. Veggies and fruit (which are often mixed in with the veggies in green drinks) have plenty of natural sweeteners in them without adding any.
Some green smoothies are veggies only, while many are combinations of fruits and vegetables. It is often recommended to eat fruits separately from vegetables in order to aid digestion. However in a smoothie this is not really a problem. The eating only one type of food at a time rule is over ridden because the two have been blended into one homogeneous food type so that the digestive tract will no longer be trying to deal with breaking down two different substances at once.
I personally very much enjoy a green smoothie made completely from veggies. I like to spice it up with salsa, add some salt, and really like it with the addition of raw eggs and tomatoes.
However an all veggie smoothie can be a bit too strong of a taste for some people to handle right off the bat. So adding in some fruit will sweeten it up and smooth it out. Bananas work well for giving a smooth texture as well as adding sweetness. Try some peaches, apples, and others to see what works well for you, and to add variety.
As for what kind of greens to use, that is up to you. Use a variety to spice up your life, as well as avoiding the overload that can come from eating only one type of vegetable for days at a time. I highly recommend spinach and kale as some of the healthiest basic veggies to include (one or the other) almost everyday. But go ahead and mix it up from one day to the next. Try different combinations to keep it fresh and interesting as well as healthy. Carrots are another great addition.
Think about combinations that you would like in a salad, and make a salad in a glass. Do not, however, add salad dressings. Most salad dressings are made with negative oils (and other additives) and contribute to health problems. If you feel you need some oil, I strongly suggest that you only use virgin olive oil, or coconut oil.
Another word of caution: Smoothies are a great way to get veggies into your diet, but be sure to eat some fresh and steamed veggies on a plate sometimes too. And don't overlook the importance of chewing.
Also remember that because the blender releases the nutrients from the food, it will lose nutritional value very rapidly. So just make enough for one meal, and drink it as if sitting down to a meal.
In fact, do sit down at the table and enjoy it as a meal. Thinking about what you are eating and enjoying it, actually helps food to digest better. Have a meal time, even though it can be convenient to slurp on the go. Slurping on the go is not nearly as healthy. A stomach in a body at rest can work much better than in a body on the go.
I like my smoothies thick and sucked through a straw. But you can add as much water as you like to make a consistency that is right for you.
The important thing is that whatever you have to do to get your greens, do it!