You have to want to wallow with bargain- hunters. You can't mind having a stranger at your elbow, shouting, "Do you want those? Can I try them on after you?" And you've got to groove on lightning pickings: See what you want and go for it, moving through the crowd with the necessary force. If you've brought your kids along -- and nine out of ten seem to -- you've got to drill them on going with the flow. Or tell them to meet you at the car in 20 minutes. That's because that annual October ritual, the ski swap, offers bona fide bargains for those who can make themselves think snow on a sunny, 65-degree Sunday. Evidently a lot of folks are blessed with such foresight; most of the swaps draw from 1,000 to 3,000 customers. Bill Printz of Potomac Ski Shop, one of the first stores to sponsor a swap, estimates that between $15,000 and $20,000 in merchandise and money changes hands at a swap. Ask Jane Stein why she's usually part of that throng and she ticks off bargains like these: Two years ago she sold her year-old $55 ski boots for $35, then turned around and bought someone else's $120 boots for $55. She bought her daughter, 10 at the time, a pair of skis with bindings for $30. Her daughter skied on them for two years and is passing them down to her younger brother. Jane also sold her son's castoff skis for $20 -- they'd had two years' use by her daughter and two years' use by her son. Then she bought her son ski pants for $5. "They're not gorgeous," she says of this last bargain, "but they're functional." It isn't just the bargains that drive skiers to the swaps. Although the sales are sponsored by a local ski shop, most are run by a branch of the National Ski Patrol; ski patrollers man the tables, work the cash registers, take a 15 percent cut on everything sold -- it's the patrol's major fund-raiser for the year -- and offer free, objective advice to all comers. "They won't tell you if something's a good buy," says Stein, "but if you ask, they'll tell you whether they think the equipment is appropriate for your level of skiing or whether the bindings are safe for your son who weighs 50 pounds." The swaps, started about eight years ago, are a happy marriage of fund-raiser for the ski patrols (needing money to buy sleds, radios and other equipment) and crowd- pleaser for the stores. The swaps create customer good will and generate store traffic at a time of year when business is slow. Almost all the gear and ski clothing at the swaps is used, though occasionally a store will put in some sale merchandise. There are skis, bindings, boots, poles, parkas, pants, goggles, ice skates and other outdoor equipment and clothing. Children's things are the best sellers and biggest draws, because kids outgrow equipment and clothing with such startling regularity. So there's greater demand among parents who pay the bills and an almost comparable supply from parents whose kids have managed to put on two inches and ten pounds over the summer. "Swap" is something of a misnomer in that you don't have to put merchandise into the sale in order to buy. Conversely, you don't have to buy equipment if you bring your castoffs. Most swaps work this way: Those who want to sell gear bring it in any time during the day, ideally before 10, and price it, although some stores will advise on prices. The ski patrol gives out a receipt for the merchandise. The seller is expected to come back at the end of the day and pick up his cash or, if things didn't go so well, his unsold merchandise. Any merchandise not picked up by the end of the day is "donated" to the ski patrol o can't expect that to sell." Most swaps are held between 10 and 4, in a parking lot. Some stores liven up the atmosphere by having a band or accordionist play music. Sometimes there's apple cider or juice for sale. The real bargain-hunters make the rounds early to catch the best buys, but since people can bring stuff in all day, the early bird does not necessarily catch all the best worms. WHERE THE SWAPS ARE

ALPINE SKI SHOP, 2964 Chain Bridge Road, Oakton. 281-1511. October 4, Massanutten Ski Patrol.

CABIN JOHN TENNIS AND SKI SHOP, Tuckerman Lane and Seven Locks Road, Rockville. 299-4141. October 11, Liberty and Roundtop ski patrols.

INNER SKI-BARTS, 8 East Diamond Street, Gaithersburg. 948-1494. October 25, privately run, one-on-one swapping and selling.

OLD TOWN SKI SHOP, 821 South Washington Street, Alexandria. 683-1510. October 17, privately run, no commissions.

POTOMAC SKI SHOP, 3610 University Boulevard, Kensington. 949-6800. October 10, Blue Knob ski patrol.

SKI CENTER, Massachusetts and 49th Street NW. 966-4474. October 18, Liberty Ski Patrol.

SKI CHALET, 2704 Columbia Pike, Arlington. 521-1700. October 10, Canaan Valley ski patrol.

SKI HAUS, 11130 Rockville Pike, Rockville. 468-7750. October 11, Ski Liberty racing team.

SKI RACQUET, 1721 Elton Road, Silver Spring. 434-2221. October 10, privately run, proceeds to United States Ski Team.