The bus passenger thought she was helping out when she leaned down to pick up a quarter dropped by a frantic fellow commuter. But when the man leaped off the Metrobus at the very next stop she had an uneasy feeling, cheked her handbag and found her wallet missing.

The "coin drop" is a pickpocket cliche, acknowledges retired Detective Sgt. Robert A. Eldridge, a 34-year veteran of the Metropolitan Police Department who specialized in pickpocket crimes and is writing a book on the subject.

"Pickpockets are experts in getting your mind off your wallet," notes Eldridge, who says the coin drop is "a very smooth operation, like a football play, usually performed by a team.

"The one who creates a distraction is called the 'stall.' He frames the mark (victim) by dropping coins while the other man, called the 'tool' or 'mechanic,' goes in after the wallet."

The tool then passes the wallet off to the stall. If you accuse the tool, he will act indignant. Meanwhile, the stall is long gone.

Pickpockets are often well-dressed, well-spoken, nonviolent professional thieves. "They try to look like someone you wouldn't mind standing next to, and some even carry schoolbooks to assimilate into the environment," Eldridge says. "Most are courteous, but at the same time they are helping you, they are helping themselves to your valuables."

Since "congestion causes distraction," pickpockets usually work in crowds. Bus stops and stations are particular favorites. Some learn their craft in jail or in a high-school lunch line, while a reported elite hold degrees from the legendary South American School of the Bells.

Summer is a prime working season for light-fingered types who have easier access to overcoat-free pockets, according to a police training manual prepared by the New York City Transit Police. It explains three basic categories and methods of operation commonly used by pickpockets.

The canon - the most skillful and daring pickpockets. While this class of thief picks only men's pockets, women have had great success working as canons. They specialize in spotting the most affluent people in the most likley circumstances, such as in a crowded bus terminal or race track. Some watch their mark purchase a ticket to see which pocket contains the wallet.

Bag-openers - often wait for a victim's hands to be filled with packages so the mark is less likely to feel a tug while the wallet is being lifted from a handbag. Bag-openers often use a coat or a newspaper to conceal their sticky fingers and favor "scoring" on an escalator or stairway, or while entering or leaving subway cars or buses.

Lushworkers - the least proficient pickpockets who specialize in removing wallets and valuables from persons who are drunk or asleep.

Ancient Romans supposedly favored togas because the flowing garments frustrated pickpockets. Short of dressing like you're headed for a toga party, there are several things you can do to keep from becoming an easy mark.

"Stay mentally alert and always be mindful of the possibility of being victimized while in a crowd," says Eldridge, who carries little cash because, he says, throngs of tourists make Washington a "pickpocket paradise."

Place a pocket comb with the teeth facing up in the fold of your wallet before putting it in your pocket, advises Montgomery County Police Officer George Heinrich, who uses this tactic when he is out of uniform. "If someone reaches in to lift the wallet, the teeth of the comb grab the pocket. Also it keeps you from pulling your wallet out and spending money."

Avoid taking extra money to a movie or sporting event, and keep your purse on your lap. "In some cases you've even got to be careful in church," says Officer Heinrich, who heard of one pickpocket snatching a wallet from the purse of a woman who was at the altar receiving communion.

Carry your purse securely under your arm, rather than hanging from your hand. Always keep your handbag toward the front of your body, especially when your hands are full.

Cram your wallet down into the bottom of your handbag, where a pickpocket would have to fish for it.

Pin valuables (such as a passport) into the front breast pocket of a suit. This is usually a safer place than the rear left pants pocket, which is called the "sucker pocket." If you do use the sucker pocket button it closed, or better still, safety-pin it shut.

Keep one hand in your back pocket over your wallet when walking in a crowd, suggests Gary Handleman, 28, the Capital Centre's director of administrative operations. He says this posture has become a habit for him whenever he walks around a crowded concert or carnival.

Wear tight-fitting pants, which are more difficult to pickppocket than baggy ones. Some officers put their cash in the front pants pocket or use a moneybelt when traveling.

If you suspect you've been pick-pocketed, "make a lot of noise," says Metropolitan Police Officer Harry Hanbury, 32. "Yell, 'pickpocket!' call for the police and point to the offender, who relies on anonymity. Remain adamant until the police arrive." CAPTION: Picture, no caption, by Harry Naltchayan - The Washington Post