School's out, you say? Now it's time for life lessons, to turn from New Math to Real Math. If you're going to survive summer travel dining with your wallet intact, it's not just a matter of keeping your credit cards strapped to your waist but of doing your homework about the sneaky ways that some shady restaurants--or honorable restaurants with a few shady waiters--operate.

Herewith 10 ways to survive vacation dining:

* The best things in life are not free; you're just led to expect them to be. That bottle of wine sitting on your table unopened? The bottled water you're offered? Ask the price. Even if it's your birthday and the waiter shows up with a special bottle of wine and a corkscrew, it's not likely to be a gift. The last time I asked the price of that birthday bottle the waiter was about to uncork, it was $250. And he'd sold four that week.

* It's not just what you drink, but how much. Iced tea and coffee may come with free refills, but bottled water doesn't. Especially at a big table, watch for waiters topping off the glasses to overflowing at every sip. The water bill can rival the bar bill. Speaking of the bar bill, keep in mind that such descriptions as "excellent," "rare" and "special" typically mean "fabulously expensive." Heed the lesson of the host who ordered four cognacs for his guests, opting for the one the waiter praised most highly. It turned out to be $100 a glass.

* Nor, of course, does the food come free. When the waiter brings a small pre-appetizer "with compliments from the chef" or a plate of sweets after dessert, that's free. But if he encourages you to leave ordering the appetizers in his hands, that usually means you'll leave more money in his pocket. I've had the waiter show up with three different selections for two of us and charge each as two portions, thus charging us for six appetizers. At one expense account restaurant, when four of us were trying to decide which one or two desserts to share, the waiter seemed to be apologizing for the evening's service glitches when he insisted that he would take care of the desserts. He brought five, and charged for them all.

* You're on time for your reservation but your companions aren't, or your table isn't quite ready, and the maitre d' suggests you wait at the bar. What better way for a restaurant to increase its alcohol sales? If you don't want to be pressured into having a drink, hold your ground. "I think I'll just wait right here" can be the magic words to get you your table.

* The waiter brings the wine bottle and briefly displays it so you can read the label, but you're in the middle of a conversation and give it only a cursory glance. He pours a taste for you to approve, then presents the cork for you to sniff. Folderol. Unless the wine is very old, examining the cork is meaningless. Instead you should be paying closer attention to the label. Thus, when your wine bill turns out to be three times the price of what you ordered, you'll know that being charged for the much more expensive "reserve" or "kabinett" or premium vineyard is not warranted. You ordered the sauvignon blanc and were charged for the more expensive chardonnay? If you didn't really notice the label, at least insist on seeing the empty bottle--and hope it's the bottle you actually drank. Also, watch the vintage. Substituting cheaper vintages for those on the wine list is an easy way for a restaurant to increase its profit, and most diners wouldn't notice.

* In Europe, the tip is usually included in the bill, and American travelers learn to expect that. But we don't expect it in Miami. Yet it's becoming the norm there, as it is for parties of six or more in nearly every other city. The menu probably warns that the gratuity will be automatically added to your check, but in flickering candlelight, many a diner has missed that tiny print at the bottom of the page. If you add 20 percent on top of the automatic tip, don't expect the waiter to remind you that he's now been tipped more than 40 percent. He wouldn't want to insult you by questioning your generosity.

* You wouldn't buy a suit without asking the price, but consider this scene: The entrees on the menu run $12 to $16. When the waiter recites the list of specials, he tells you every ingredient, each spice and the cooking method. Note that he doesn't tell you the price. Is that because speaking of money is considered rude, or is it because the veal chop is $30? If you don't ask, you might find an expensive surprise at the end of the meal.

* Computers are scoundrels. At a major downtown restaurant, the smoked-salmon appetizer appeared on the bill at the price of the salmon entree. "It's the computer's fault," said the waiter. Was the computer fired? Made to mend its ways? Reprogrammed? Two weeks later, that devilish computer was still ringing up appetizers as entrees.

And there are subtler overcharges. Restaurants throughout the country use a computer program called Comus to compute the tax on your bill. One key does it. But that lazy Comus doesn't bother to distinguish one charge from another. Thus if your party's had the tip automatically added to your bill, Comus will charge you tax on the tip, too. You could be paying up to 13 percent tax on your food rather than the required 10 percent. Multiply that by a couple hundred bills a day, and Comus has a profitable little mistake going.

* In the grocery business, some unscrupulous cashiers used to add the last customer's purchases onto the next one's tab. The cashier could pocket the double charges. In restaurants, it's more likely that a bigger bill is substituted for a smaller one. Even bolder, some restaurants have been known to run the bill twice, which might not be noticed on a monthly credit card statement if the charges were submitted on different days. More modestly, some waiters change a 3 to an 8 on the tip, or add an extra digit where it might not be noticed.

* Then there are the grand schemes. One New Yorker took a group of clients to an expensive restaurant for a holiday dinner in December. Two or three months later, the restaurant called him and, apologizing for the bother, said he had signed the wrong credit card check. Would he mind signing the right one? Sure enough, his credit card statement didn't show the charge. Of course he would make good on it. Until he remembered that that night he'd paid cash.

Phyllis Richman is The Washington Post's restaurant critic. This list is adapted from her forthcoming book, "Murder on the Gravy Train" (HarperCollins).

CAPTION: Caviar emptor: Let the diner beware, especially when the bill is added up.