I had a comment posted a couple posts back by chrikaren with this very question.
Unfortunately I don't think there is much that can increase the average tip per delivery. Most people always tip, but a few never tip. And most people that do tip, always tip the same amount.
The one thing that I think affects my tips the most is how we answer the phones. I try to answer as many phone calls as I can when I am in the store between deliveries. I sincerely believe that most people have decided how much they will tip by the time they get off the phone, right after hearing their total price.
So if I can be super courteous and helpful on the phone, and sound excited to take their order, I think it helps. Sometimes I can even joke around ... if the customer keeps asking people in the background what they want, I'll laugh and say "You're really stuck in the middle here, huh!" And usually they laugh too.
Another thing is quality. Sometimes customers may tip less (or order from somewhere else) if we mess up their order. If they ask for no green peppers on their deluxe pizza, make sure that's how it is made. If they want their wings well done, be sure to put them through the oven a little extra.
Finally, as a driver, I have some effect on my tips. I'm clean and neat and cheerful at the door. I'm as prompt as I can be, and I will sometimes pick up a newspaper laying on the step. I'll say hi to the kids, smile at the dog, help them hold the door, all of those things help a little.
One thing I can't change is my looks. We had a driver named Amy, an attractive young girl, and she was the one person I worked with that almost always made more tips than me. She's over at Marco's now, and I'm sure she still gets tipped well.