I've started carrying my own containers with me everywhere I go. I keep a bag of them in the car so if I realize I need to stop to get something, I don't find myself needing to use non-reuseable packaging. I keep 3 cloth bags in my purse (they roll up really tiny and I keep an elastic band around them, which I pop into the bag when in use), and some tupperware containers in my trunk. At first I was asking places if they would mind if I brought my own containers "next time" and wasn't getting very favourable answers... "We can't, as it could contaminate our product, which we guarantee", etc. So now, I just waltz into any store carrying my own containers and as I place my order, I just hand over my container like I've done it a thousand times. Much more favourable results! The sales people are a little unsure, but I think my confidence that it is just fine affects them! More times than not, I get my food, in my own container, without much hassle.
I get some looks from other people in the store -- especially at the deli where they're all standing around waiting their turn, watching what other people are ordering. But if they see it a few times, maybe they'll consider doing it themselves!