Though I generally agree with Randall that protein shakes are not to be trusted, I keep a can of Tera's Whey on hand for convenience. Sometimes I just don't feel like eating, but I know it's going to be a long time before I have the chance again, so a quick protein drink keeps me going without making me feel like I've had to force feed myself so much. I found the powder at Whole Foods. They also carry a purely grass-fed version, but I've only seen it in unflavored (I have chocolate - 4 carbs per two tablespoons, and it's organic).