20% on good service; if they're fun, knowledgeable and enhance the overall experience even more. Bad service gets less, and they'll know why. If they don't, well then that explains why they're stuck with crappy shifts at a middling restaurant. Also, depending on the place and the quality of the meal, a pitcher of beer for the kitchen staff or a bourbon for the chef may be in order. I know myself and the crew appreciate it, and if you're a regular those bourbon points add up.
After ~20 years in the industry, and doing both sides of the house I can tell you that for 3-4 bucks an hour its generally well deserved. And as stated above, some of the people are also splitting out to a host, captain/front waiter, back waiter, bussperson and bartender. That 20% gets chewed up pretty quickly. In a busy mid level restaurant those people run their asses off; in a high end place the level of service is higher; although the work is split, going through pre-service and studying the menus, knowing the food, ingredients, where its sourced, etc takes a reasonable amount of effort away from the restaurant. Not every place will have a sommelier either, so a decent grasp of wines (and now specialty cocktails) will be in order. You also end up being a mediator between the customer and kitchen staff who also get their fair share of shit shoveled their way. I've heard numerous times "oh just pick up a job waiting tables" to make some quick easy cash....which can be true, but to make reasonable cash takes a lot of running at a bisy spot, and to make mmmoney at it, real money, takes a lot of time and effort moving up the chain to a place where 70 or 80k is in the cards.
You might be a polite, fair to low maintenance customer, but make no mistake; two tables over is Mr. Richard Cranium, his abbohrent wife and two demon spawn children running that person ragged and being rude to boot, taking the word "server" as if they're plantation owners in the 1800's. To add a little lemon in their eye, after numerous trips back to the kitchen, a million (usually stupid) questions, snide remarks within earshot, refilling drinks for the brats even though the glass is half full and sending food back even though they got exactly what they ordered, they're left with pocket change and a sneer as they leave a veritable shitstorm of a mess behind, not to mention they might as well have brought sleeping bags as they camped out for hours and kept the table from being turned over.
For those that can't get over the concept of tipping, there's a couple of choices:
1.) Stay home. Your server will appreciate it.
2.) Fake a resume and get yourself a serving job at a busy, upscalish type place and see what its like. OK...don't do this; you may end up stuffed in the ice machine with an oyster fork lodged in your temple after your first shift with one of your coworkers on their way to the clink.
3.) Move to a place where the additional cost of paying the servers is figured into the cost of your food.
4.) If none of those sound amenable refer to Rule #1.
*if you happen to be part of the Cranium family or close relation, keep in mind that these people are preparing/handling your food. :)