My general policy is to not tip on take-out/carry-out, unless there's effort put into packing, cutlery, etc.
For example, there's a nearby gourmet pizza place where the pies are $30-35. The pizzas are just boxed and I take them away. I don't ask for plates or napkins or red pepper. I gave my assistant my card and she gave them a $10 tip on a $70 bill. Suffice to say I wasn't happy with that.
For the record, I'm a generous tipper at restaurants (I tip on tax, too) and tip delivery guys well (I would've given these pizza guys probably $100).
And if I go to a fast-food takeaway place (like Panda Express/Yoshinoya/San Sai) I definitely NEVER tip.
What do you do?