Maltodextrin with Crystal Light Lemonade rocks! It's easy and cheap to mix up a bottle with 300 calories, 'dex is about $1/pound and doesn't sludge out in the bottle. I haven't tried adding protein to it yet. I was using Hammer Perpetuem and Sustained Energy for double centuries and training. Both worked well for me, 'Dex is the main ingredient in both.

Arnie Baker Cycling is a good source for info on nutrition.