If you're going for store bought juices, stay away from the really high Sugar stuff with no fibre, like Sunripe Apple, or those "apple orange peach mango" drinks.
From what I understand, vegetable based drinks like v8 (especially with extra fibre) are excellent, though not too much at a time (and maybe dilute it some at the beginning).
What do people here think of steamed peas and carrots? Providing you drink the water they were steamed in of course.