"It's not tipping I believe in. It's overtipping. That's my philosophy."
We order carryout fairly often. I always tip at least 20%, even if it's just a $5 order at Sonic. Mostly it just makes me feel happy to do it because I know the poor server/bartender who got stuck handling my carryout isn't expecting it. If the service at a restaurant is consistently bad enough that I don't feel a tip is justified, I usually offer a final tip of 10-15% and simply don't return. There are too many good places to eat to suffer poor service.
Also, I consider tips payment for services not yet rendered. IOW, they're not for this visit, they're for the next one. If you go to the same places often and tip well, you get treated well by the servers, not only on carryout orders, but especially those times when you eat in the restaurant.
I live in a small town, and I'm pretty much on a first name basis with most of the servers at the 5 or 6 restaurants from which we order, so it's reasonable to expect to see the same folks over and over.