I'm sure that I'm doing it wrong somehow, but I typically tip 10% for carryout that involves "significant" effort in packaging. Sushi or Chinese food with sauces and rice boxed separately from the main dishes gets a tip. Pizza tossed into a box does not. I also tip 10% at a buffet restaurant since there is some service involved, but it's not quite "full" service. More typical restaurant service usually gets 20% from me.

I realize it's completely arbitrary, but so is tipping culture in general.