I talked to spokespeople from Visa, MasterCard, American Express and Discover to get the scoop on their policies, and with Fair Isaac, creators of the FICO credit score, for details on how cards really affect your score.

The myths and the reality:

Myth. 1: You can stop unsolicited credit card offers by sending them back in the postage-paid envelopes.

Judging by my e-mail, some of you have developed a hobby trying to irritate credit card companies. You write “take me off your mailing list” repeatedly over the unsolicited applications they send you, then stuff the paperwork into the postage-paid envelope — sometimes adding other junk mail to increase the volume and cost the issuer more in postage.

Sorry, but all your efforts are for naught. Yes, you might cost the credit card company a few pennies, but it would cost them far more to track down your name on their mailing lists and remove it, so your envelope just winds up in the garbage.

Myth. 2: Merchants may require identification, such as a driver’s license, when you pay with a credit card.

Merchants’ agreements with Visa, MasterCard, American Express and Discover specifically forbid them from requiring identification. Your signature is supposed to be enough.

Furthermore, merchants’ contracts with Visa and MasterCard are supposed to prevent them from even asking for ID. American Express and Discover don’t prohibit asking but strongly discourage it.

Merchants typically ask for ID because they’re trying to reduce their own fraud costs. But if a clerk memorizes or writes down vital information from your driver’s license — your address or date of birth, for example — you’re the one who could be at greater risk of identity theft.

Myth. 3: You can deter identity theft by writing “Ask for ID” instead of your signature on the back.

See above. You’ll certainly deter use of your card, because merchants aren’t supposed to accept one that’s not signed on the back, and that could affect you as much as any thief.

Myth. 4: No-limit credit cards allow you to buy whatever you want.

Most credit cards come with credit limits, but some cards advertise having “no preset spending limits.” With high-end Visa cards, for example, customers are allowed to exceed their credit limits; with traditional American Express charge cards (the green, gold, platinum and black versions), there is supposedly no preset limit at all.

Myth. 5: Your credit card account isn’t opened until you activate it using the issuer’s toll-free number.

Several readers have changed their minds about opening new credit cards after they’ve applied, then asked if they could undo the damage to their credit scores by not calling to activate the card.

Sorry, but the ding to your credit scores — typically 5 points or less — happens as soon as the issuer pulls your credit reports, which is usually within seconds of receiving your application. The account shows up as active on your credit reports shortly after the card is approved.

You do need to call the activation number, though, if you ever want to use the card. That number is typically listed on the removable sticker on the front of your card when it arrives in the mail.

If, for example, you’re a good customer who typically spends $3,000 to $5,000 and you want to charge a $50,000 luxury car to your card, you’d be smart to call Amex first to make sure the transaction would be approved.

If, on the other hand, you’re in possession of an American Express Centurion Card, a black version that usually isn’t even offered to folks who charge less than $250,000 a year, you probably needn’t worry about getting approval for the same transaction — unless “your people” forgot to pay last month’s bill.

Myth. 6: If you pay your credit cards in full and on time, you don’t need to worry about your cards’ effect on your scores.

Myth. 7: High credit card limits are bad for your credit scores.

I’ve heard this one repeated by folks who should know better, including mortgage brokers and other lending professionals.

Here’s a tip: If you’re told the reason your credit scores aren’t higher is because you have “too much available credit,” that pretty much means you have great scores. Typically the only reason you’d hear this “negative” is because there’s nothing else wrong with your credit.

You certainly shouldn’t ask a credit card company to lower your credit limits or shut down cards, since either action could hurt your credit scores, unless a lender specifically requires you to do so as a condition of getting a loan. Even then, you should try to keep your oldest and highest-limit cards open.

But you also shouldn’t run out and open a bunch of new credit card accounts without considering the consequences. Each new account application can ding your scores and represents another set of rates, due dates and terms you’ll have to track. Apply for credit sparingly, and don’t worry if your issuer rewards your good credit habits with a higher limit. It knows you can probably handle it.

Myth. 8: A credit card company can’t change my rate unless I mess up.

Myth. 9: Rewards cards are pretty much the same.

This myth takes different forms, including “the best rebate you can get is about 1%” or “you have to pay an annual fee to get a rewards card” or “the rewards aren’t worth the effort to redeem.”

It’s all bunk, said Arnold, of CardRatings.com. Consumers who shop around will find big differences among rewards cards. Today, the best cash-back rewards cards have no annual fee, and you should expect a rebate in excess of 1%.

How to be smart with your credit card?

1. Apply for the right card

Universities and alumni associations rake in big bucks steering you to certain credit card companies, which will flood you with offers via e-mail, snail mail and campus kiosks.

But that doesn’t mean those issuers will give you the best deal. A university-branded card may tout a low initial interest rate, for example, that quickly jumps to 19% or more after a few months.

Even worse are the so-called subprime credit card vendors that stick you with big upfront and annual fees that can eat up most of some cards’ available credit.

Check out the student cards offered by sites such as CardRatings.com and Bankrate.com or the best overall values identified here on MSN Money. Don’t get sidetracked by rewards programs, low introductory rates or other trimmings.

You want a card that:

• Charges no application fee or other upfront fee.

• Has no annual fee.

• Comes with a decent interest rate; 14% to 17% is typical for a student card. (Avoid a card that offers a rate “as low as” a certain percentage, because you’re likely to wind up with a much higher one.)

• Reports to all three credit bureaus.

That last point is important. If your card isn’t reported to the bureaus, it’s not building your credit.

How do you know? The major issuers that specialize in student cards, such as Citi, Capital One and Discover, report to all three. Before applying for a card from a credit union or small bank, though, ask in advance whether your account would be reported to Equifax, Experian and TransUnion.

Also: Apply for one card at a time, and let several months to a year pass before you apply for another. You need only one card to start building a credit history; a second card will continue to help, but if you apply for many more you’re just increasing the chances you’ll run into trouble.

2. Study the lit

Your card will come with a fat packet of information providing the details of your card, including benefits and fraud protection. Some of it is important (yes, there will be a quiz), such as:

• What are your interest rates and fees? We hope this information will be academic (more on that in a moment), but you still need to know. You’ll typically have three rates: one for purchases, one for balance transfers (debt transferred from another card) and a third for cash advances. When you make payments, they’ll go toward the lowest-rate balances first, so your higher-rate balances will continue to accrue fat interest charges. Also, check out the fees for paying late or charging more than your limit. They’re ridiculous, so avoid them.

• What’s your credit limit? Speaking of ridiculous, your limit may be laughably low with your first card; $500 or less isn’t uncommon. What’s more, you don’t want to charge anywhere close to that amount. The best way to build your credit is to charge no more than 30% of your limit, so no more than $150 in any given month on a $500 card. Remember, you’re using this card to build your financial future, not finance your lifestyle.

3. Your card is not a toy

• Misuse your plastic and you could pay the price for years. Some things in particular you’ll want to avoid:
• Taking cash advances. Your card is not your bank account. Cash advances come with high interest rates that apply immediately (there’s no interest-free grace period, as there is with purchases when you pay your balance in full every month). Find a cheaper source of cash.

• Paying late. You’ll trigger a late fee ($30 to $40, typically) and possibly a higher interest rate. Worse, if you skip a payment entirely, you’ll trash those credit scores you’ve been trying to build. A single skipped payment can knock 100 points off your scores, and the damage can linger for years.

• Getting anywhere close to your limit. The less you charge, the better, but your scores will really take a hit if you let your charges creep above 80% of your limit.

• Paying the minimum. This is such a bad idea it needs its own section. Read on.

4. Pay your balance in full — always

Your first statement will show you how much you’ve charged by the statement closing date as well as something called a minimum payment, which is typically 2% to 3% of your total balance.

If you want to wreck your life, pay only the minimum due each month.

Seriously, carrying a balance is an awful, expensive habit, and it leads a lot of folks into financial hell. When you pay the minimum, you’re setting yourself up for years of debt and total payments that pretty much double the cost of whatever you’ve purchased.

If you pay only a 2.5% minimum on a $2,000 balance at 15% interest, for example, you’ll be paying off that debt for about 15 years at a total interest cost of $1,758. You don’t have to be a finance major to know those numbers suck.

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