I had a LOT of trouble doing that - fact is, I work out of my car, never seemed to remember to take a cooler for the left-overs (did I mention I'm not a morning person?
)... I'm finally at a point where I see it as a "health payment" if I pay for the whole thing, but only eat half. (I used to call it a "health tax", but Joe Selch led me to re-evaluate the name!) The Health Payment is what I pay when I spend the same amount of money to get a sandwich and drink as I would have spent to get a sandwich, drink and FRIES. So I pay the same and don't get the fries...
This idea, plus exercise, has me almost 9 pounds less than I was last month.