i always tip 20%... even more if they were spectacular, less only if they really deserved less. but i'm one to order water with dinner simply because if i'm thirsty i need water to hydrate lol. it's silly to think that way but i'm sure there really is a correlation. it'd be interesting if somebody studied the tipping habits based on age, gender, race, etc. as well as tips correlating with what they ordered etc.
In my own experience here is what I found
but there are always exceptions:
Teenagers who don't work and are by themselves - tip poorly mainly because they don't have a job and don't know how it is.
College students - really varies from what I saw. Again if they don't work, they tip poorly. Some might try to be less of a hassle if they can't tip you more but most will try to tip you decently.
Families - Depends on who is tipping. Generally the women tipped less especially if their husband seemed to think you were cute. Also, where I lived the women seem to think that everyone owns them something and that they are to be worshiped.
Couples - Generally I had good experiences with couples unless they were teenagers. The guys would usually pay and wouldn't want to seem cheap to their date. Although if they were fighting, you're probably not going to get a great tip unless they make up. You probably don't want to be at the table much if they are fighting anyway. They may try to pull you into the arguement.
Free-loaders - Usually you can tell them by their attitude before they sit down. They order just water and will order something whether it is expensive or not can complain about it until they get it for free. Don't look for tips from these people.
Smokers/Drinkers - Usually pretty laid back and love to chat. Usually great tippers.
Regulars - Usually tip well and they should! If your server knows you by name and can guess what you're going to order,
tip them!!!!!!!!!! If not you might soon find that the server's section "is full" when they greet you at the door.
I tip pretty generously because I can afford to and I know how it is.