Delivering food for a delivery restaurant is a tough job, so the experience will better for everyone involved if you treat the driver as courteously as you would treat anyone at a sit-down restaurant. It won't hurt to tip them, either, since they're bringing the food straight to you.
Before you call in an order to a delivery restaurant, make sure you write down everything everyone wants. That way, the ordering process will go a lot easier, you'll be more likely to get what you want, and the restaurant can start preparing your food a bit quicker.