Most Americans (myself included) also have no idea about this sort of stuff, and there's often debate about it. Outside of restaurant waitstaff, the only times you tip are for what I think of as "personal services", like barbers/hairdressers, masseuses, etc., and hotel staff, from the bellhop (porters) to the housekeeping service. Also if someone carries your bags at the airport (or train station).

First, the generally recognized standard for waitstaff is 15% and generally you round it off to the closest $0.25. (Whether it's 15% before or after tax is also a debate, but there's really not going to be a lot of difference there.) Tip more (up to 25% without being a showboat) if the service was exceptional, less (possibly down to 0%) if it was notably bad. These tips are generally only given to full-service waitstaff, where they take your order, bring your food, and refill your drinks. (Bartenders tend to fall in this rule as well, but you'll often see them tipped a good bit more. That may be because their patrons are drunk.) If the only service a waiter provides is to bring your food, as when you order at a register but your food is brought to the table, there's no need to tip. Sometimes buffet-style restaurants will have waiters that deal solely with drinks. I tend to tip $1 (per diner) there, regardless of the bill, but that's hardly a rule. As you can see, there are a lot of ill-defined points here, and this is, by far, the most concrete tipping standard in the US.

As for other services, even an American wouldn't get much of a second glance for failing to tip anywhere else. A foreign tourist would get even less notice. So if you want to be safe and cheap, you could easily get by without tipping anyone else.

Keep in mind that waiters rely on tips. Their salary is generally below the legal minimum wage with the expectation that it will be made up for with tips. This is sometimes the case with bartenders as well. Tipping in any other situation really is a gratuity. Some might reasonably consider it a bribe for exceptional service. As such, you might want to consider tipping people other than waitstaff in advance, but that might also be considered a little gauche.

That said, there are a number of tipping guides on the internet:

http://www.itipping.com/tip-guide-travel.htm
http://www.getrichslowly.org/blog/2006/10/12/basic-tips-on-tipping-how-much-and-to-whom/
http://www.tipping.org/tips/us.html
http://www.tip20.com/tipping-standards
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Bitt Faulk