SALEM, Ore. (CNN/Money) – "How can you tell if a car salesman is lying?" the joke goes. "His lips are moving."

To be fair, car salesmen may not be telling a bold-faced lie when they say they're giving you a great deal. Neither are credit card companies necessarily dishonest when they tease you with low introductory rates, then jack them up later. Didn't you read the fine print?

Omissions, exaggerations, confusing language, small print and white lies are used all of the time to ring in a sale and pad the bottom line – and we fall for them.

This is one of the most common lines used in car sales, said Philip Reed, who worked as a car salesman for three months to prepare for his role as consumer advice editor for Edmunds.com and author of "Strategies for Smart Car Buyers."

"They are presenting themselves as experts guiding you through a difficult maze of transactions and contracts," said Reed. "If you haven't done your research ahead of time you're likely to take the car salesman's advice, even though they have a vested interest in getting you to pay a higher price and getting you to buy now."

Buyers fall for one-liners and other sales strategies all the time. "We're losing money on this" is a powerful fib because it gets buyers to ease up on their negotiations. Another popular line is, "My boss will never accept this offer," because it makes buyers believe the salesperson is on their side of the bargaining table.

Your best defense is to find out as much as you can about a given car and its price before you even think about showing up at a car dealership. "The more knowledge you display, the less likely they are to pull out these tricks," he said.

When it comes to credit cards, there is no such thing as a fixed rate.

"Basically, credit card companies can write their own rules," said Linda Sherry of Consumer Action, explaining that issuers can change their interest rates – even fixed rates – at any point as long as they send cardholders advance written notice.

If you pay late, issuers can automatically increase your rate. According to Consumer Action's 2004 survey of issuers, 31 percent of issuers will levy a penalty rate after just one late payment, while more than 35 percent said that two late payments in a period of six months will trigger a penalty rate. The average penalty rate: 22.9 percent.

Sometimes it's not enough to pay your credit card bill on time. Increasingly, issuers are penalizing cardholders who are late paying other creditors. Nearly half of all issuers have what's known as a universal default policy, said Sherry, and they don't all disclose this in the fine print.

"You may not realize it, but paying your mortgage late could trigger a penalty rate on your credit card," said Sherry.

Long distance companies advertise rates of 5 cents or 7 cents a minute, said SaveOnPhone.com CEO Bill Hardekopf, but don't mention that you'll pay a monthly fee of $4 to $6 a month to get that low rate. If you're the average customer making 120 minutes of calls a month, that fee brings your per-minute rate up to about 10 cents a minute.

That rate goes up even more when you count fees and surcharges that sound official but aren't. AT&T charges 99 cents for a regulatory assessment fee that covers the "costs associated with interstate access charges, regulatory compliance, advocacy costs and property taxes," or in layman's terms some of the costs of doing business.

MCI's carrier cost recovery costs long-distance customers 99 cents a month and local customers 1.4 percent of their bill. There is also a 2.3 percent property tax surcharge.

The Federal Communications Commission allows -- but doesn't require -- carriers to charge up to $6.50 a month for a federal subscriber line charge to help pay for the local network. "It's really just part of the basic service," said FCC spokesperson, Mark Wigfield.

Frequent flier programs have made it easy to accrue frequent flier miles without ever stepping on a plane. "But what airlines won't tell you is that when you try to redeem your free ticket you're competing with 40 million other frequent fliers for the same ticket," said InsideFlyer magazine's Randy Petersen. "We all want to go to the same place and there aren't enough upgrade seats to go around."

Unless you plan to use your miles for some of the less popular routes, he said, you'll probably need to cash in more miles for an unrestricted award. Even then, there are fees and surcharges, including Peterson's biggest pet peeve, the fuel surcharge. "Free really isn't free, no matter what you read," he said.