For the last three years or so, I have been an on and off Pizza Delivery Boy in a small, upper-middle class town in Massachusetts. Perhaps another node will afford me better oppurtunity to detail why someone would enjoy delivering pizzas for a living, as for now I will say I like the people I work with, and the money's not bad.
The money is not bad because in addition to my hourly wage, which is modest but respectable after years of employment, I find it easy to maintain my lifestyle off of tips. There are often cases however where people just don't seem to understand how much the waiters and waitresses, delivery boys and delivery girls, etc. etc. of this world rely on this source of income. It is for that reason I wish to describe in detail

The Art Of The Tip.

1)Put yourself in their shoes.
Waiters and Waitresses, not to undermine their positions, have it easier than delivery persons when it comes to being tipped. This, to me, is a psychological fact. In a restaurant, the relationship between customer and server is more related. The customer is seated, then greeted, then discussion ensues about the food, then the food is brought, etc. The entire meal in a restaurant consists of constant reassurance and service, and by the end of the meal you are, not to overdramatize, more entwined to the person who has been serving you than before you sat down. With this minimal weight as it is, a customer is inclined to take things in to account to give their server a fair tip. In a delivery situation, there is less of everything. The food arrives, and this is where you meet your server for the first time. You pay your server, and your server leaves, and doesn't come back to ask you if you would like anything for dessert. It is a far less involving relationship between customer and server, and thus the customer is less inclined to supplement the server for his/her service.
I always say, in tipping, put yourself in your server's shoes. In a restaurant, 15%-20% of the bill is considered an adequate tip. I encourage paying slightly more if your server seemed to apply the neccessary effort to make your dining experience pleasurable. People generally do not go by the 15%-20% assumption when ordering delivery however, but even so - most of the time far less food is ordered for delivery, and 15% of the bill can be very small when your boss doesn't compensate you for your gas. In this case I always feel a customer should put themselves in their server's situation, and ask themselves:

"What is the least amount of money I could walk away from this with and feel satisfied?"

In their own shoes, perhaps the question should be, "What is the most amount of money I can reasonably part with?"
If you would prefer a specific figure, this Pizza Delivery Boy always feels that a $3 dollar tip has respected the effort it takes to deliver someone's food. Anything above three dollars is considerably generous. Of course, if your Delivery Person is bringing you two hundred dollars worth of food, three dollars will most certainly offend. Always be aware of other people's feelings.

2) Tipping someone you know.
The shadiest of all tipping subjects, I suppose. I would say in a restaurant/delivery situation, it would be the right thing to give the largest tip possible. Doing so supports your server and shows a respect for their way of earning a living, just as you would hope they would respect and support the atomosphere you work in, whether it be an office building or a sub shop. If however, your server has told you not to tip them in this instance, take their word for it. You wouldn't want to make things weird.
This section should also cover tipping people you know in general. For instance, if your friend's son comes over to help you fix your computer. Do you tip him for his work? Do you tip a colleague you had drive you to the airport? In these instances, I always say to tip if it's expected. Don't tip anyone who's doing you a favor, but if you're asking them to do something slightly out of the ordinary that would take time away from any other work schedule, perhaps a tip is in order.
In closing, tipping is an act of generousity, but it is still a very important source of income for many people, and to neglect that in your actions would be wrong. Some people live off of tips, and some of them actually connot live without them. This is a very weighty subject to me, personally, and I always tip as well as I can. And so, I leave you with a motto of mine, a work motto, a tip if you will excuse the pun, that I believe sums up all of the components of good gratuity:

Don't order the food if you can't afford the tip.