I have always had trouble eating enough and keeping my weight up. So I take Spiru-tein shakes made by Nature's Plus. I get them over the internet at www.naturesstore.com
Do a search for spirutein to find all the flavors available (I keep kosher and many flavors are kosher, but not all, so I haven't tried all that I would like to)
or you can go to your health food store for a more limited selection. They're kind of expensive, but worth it. The website has free shipping on orders over $75. I save at least 10-12 bucks per 2 lb container so it's worth it for me to just purchase in bulk like this.
Spirutein has tons of vitamins, and lots of nice trace minerals (choline, Inositol, etc). And it's a real protein, caloric and energy boost.
The chocolate is definitely an acquired taste (kind of strange). But the Vanilla, Peanut Butter and Chocolate, Banana, Cappuccino and Strawberry are really delicious. I would recommend using more milk than the directions say to, though, as they often too thick if the directions are followed.
They are much much better if made in a blender. Whole milk works best--don't use soy milk (spirutein's a soy protein product) or it gets really gloopy. Almond milk is good, or Rice milk. Once again, I always use more milk than it says to. And you can change it up: add nuts to the blender, frozen banana or frozen berries, or yogurt.