One problem that I've had in the past with making smoothies ahead of time is getting the right consistency because many smoothie recipes rely on ice or frozen fruit to thicken them and thus are not great after several hours in the refrigerator.

I've been playing with a make-ahead recipe that includes oatmeal and chia seeds to thicken the smoothie, which means you can make them ahead of time without sacrificing thickness: http://www.theyummylife.com/Oatmeal_Smoothies