(MONEY Magazine) – Tipping may be a 200-year-old tradition, but it hasn't lost its power to puzzle. Sure, the basics seem straightforward enough, but real-life problems still come up all the time. We don't have to tell you, for example, that you don't tip professionals, managers, owners or artists. Yet hairstylists, whom we tip religiously, can fall under any of these categories.

The exceptions and questions become more frequent on the road, where service workers appear at every juncture. Are you supposed to tip the driver of the rental-car shuttle, for example? What about the airport skycap? (No for the driver; yes for the skycap.) Train porters expect to be tipped, but flight attendants will probably be insulted by the gesture.

If all else fails, don't be embarrassed to ask what's appropriate. Marjabelle Young Stewart, author of The New Etiquette (St. Martin's Griffin), says she regularly asks management, other guests or sometimes the employees themselves.

But the fact is, even the most graceful tippers find that their experience and instincts sometimes fail them. So we've gathered a few of the perplexing dilemmas you're likely to confront while traveling, starting with the basic.

Didn't I already pay for this? If it seems to you that every other person who performs a service is looking for a little bonus pay, you're right. Some 30 different groups of service professionals in the U.S. supplement their income with gratuities, making tipping--by conservative estimates--a $16 billion industry, says Michael Lynn, a professor at the Cornell University School of Hotel Administration. You may be paying $300 a night for a hotel room, but you are still expected to tip the bellhop at least $1 per bag, room service 15%, the chambermaid $2 a day and the doorman $1 for hailing a cab ($2 if it's raining).

So deal with it--and budget accordingly. A couple spending a week in a hotel can easily add $200 in tips to their tab. Tips can add up to as much as 20% of the cost of a cruise. (Though the rules are similar to those on land, guidelines are provided by most cruise lines in their information packets.) The services of a golf caddy can run as much as 25% of the greens fee, while ski instructors get up to $100 for a full day of private instruction. Guides on everything from bus tours to Alaskan hunting expeditions expect an extra 10% to 20% at the end of a trip (but don't tip guides at national parks or other government sites).

I've been a huge annoyance. Couldn't you have asked for the sauce on the side before the food came? And was a romantic bistro really the right place to bring your daughter's soccer team, still in cleats? Let's face it, everyone can be high maintenance at times--and we should pay for it. Kim Stahler worked as a waitress for 12 years before launching the Stained Apron, a Website that is devoted largely to food servers' gripes (www.wso.net/apron). Bad tippers in general are regarded with disdain, she says, but customers who cause servers to work triple time and then leave a wimpy tip really stand out. "I once waited on a man who ordered king crab legs, but because he had only one arm, I had to crack them open for him one at a time," Stahler recalls. Needless to say, she wasn't pleased with his 10% tip. "When you go beyond the call of duty, you expect them to kick in a little more."

While we all can be annoying in our own way, several common situations demand at least 20%. Staying at a table longer than usual in a crowded restaurant means wages lost for the waiter. "Your tip should reflect that," says business etiquette consultant Ann Chadwell Humphries. When your meal costs considerably less than the restaurant's average, or something is served on the house, 15% doesn't always cut it either. Rowdy groups and small children who make a mess or require special favors should also bump up the gratuity. The same reasoning, of course, applies outside the dining room. Rethink your tip when you share a taxi and make several stops or when you ask a bellhop to carry especially heavy luggage.

That was terrible--but it's not your fault. The food was inedible, the busboy spilled water on you and an air-conditioning problem did nothing for the ambience. Yes, these things happen. But at some point shouldn't you skimp on the tip? First ask your waiter to get the problem corrected, argues Mary Mitchell, who writes the syndicated etiquette column Ms. Demeanor. Says Mitchell: "If he is earnest about making sure you like your food or about fixing anything else, he probably deserves 15%." If he fails to meet that standard but doesn't deserve all the blame, Mitchell suggests leaving 10%. When the service is just horrendous, most experts suggest you leave a symbolic tip of 5%. If you leave nothing, the server may think you just forgot--and the old trick of leaving just a penny is, well, tacky.

Would you care for a bribe? We've all had the fantasy: You slip the maitre d' some cash ("I can think of 50 reasons I need that table"), and he shows you to the corner banquette that the poor suckers in line have been eyeing for an hour. But real life doesn't work that way. While some Bogart wannabes insist that a request for a primo table gets heard only when accompanied by a $20 handshake, the best maitre d's say they'd rather have no tip at all than an up-front bribe. Cash in advance is apt to insult their professionalism--and won't necessarily help your situation anyway. "If I can't accommodate someone who has no reservation or wants a specific table, it's simply because they can't be accommodated," says Alfred Hezekiah, a maitre d' at New York City hot spot Jezebel. But that's not to say that you should forget about tipping him altogether. If you're a regular customer, says etiquette expert Stewart, your best bet is to--discreetly--give the maitre d' $10 or so after you've been shown to the table or as you're leaving the restaurant. Next time you visit, your gesture is likely to be remembered and rewarded. "My best customers always tip when they're leaving," says Hezekiah. "It makes them more familiar, which can help when they come in and, for instance, don't have a reservation."

The concierge goes all out--and fails. You probably know that when the hotel concierge snags you seats for the hottest show in town you should tip him 10% to 20% of the cost of the tickets. And you may also know that his reward for smaller courtesies, like making dinner reservations, should be $5 or $10. But even the best-connected concierge can have problems now and again. So when he tells you that, despite two days of work, you won't get those World Series tickets after all, do you still slip him $20, or simply thank him for his efforts and save the money for next season? "If someone has clearly put in the time, they still should be rewarded," says Mitchell. But that doesn't mean you need to be as generous as you would have been--a token 5% to 10% will do.

Traditionalists say you should tip the concierge all at once at checkout--and put the money in an envelope. But Bryan Szalonek, concierge at the Westin River North Hotel in Chicago, begs to differ. "If I'm tipped right away, I'm more apt to go the extra mile the next time," he admits. As for the envelope, "Get over the notion that it's insulting to hand over naked cash," says Humphries. "It's not!"

Someone else is picking up the tab--and he's a cheapskate. Like most people who have ever waited tables for a living, Jennifer Keiser firmly believes in the 20% tip. So when she started dating her future husband Mike five years ago, it was with great restraint that she said nothing about his leaving only 10%. "He didn't know how much to tip, but I didn't want to embarrass him," she says. Her solution? For almost a year Keiser would peek at the check and sneak extra cash under her plate. (Finally, she confronted him; Mike's now a strict 20% tipper as well.)

Alternatively, you can always try a less risky move. "I insist on at least getting the tip," you can say and then you are free to leave as much as you'd like. In most cases, however, you'll probably have to defer to your host--and, like Keiser, teach him the rules later.