Some airline passengers, it seems, are quite willing to sit wherever the check-in desk puts them; but many others have specific likes -- and dislikes -- and they make sure they get a seat that pleases them.

"Many travelers familiar with certain routes identify certain seats," says Rod Strickland, director of customer services planning for United Airlines. Often they are business travelers, and they ask for the same seats every time they fly.

Most major U.S. airlines (and foreign airlines flying from this country) now permit passengers to choose their seats and pick up their boarding passes 30 days in advance. (One that currently doesn't, Northwest Airlines, expects to have advance seat selection by the end of the year; and USAir inaugurated advance selection this month.)

And some airlines allow passengers to select a seat at least 11 months before the flight -- among them United Airlines, American Airlines and Pan Am -- and many travelers are taking advantage of the opportunity.

If a seat location is important to you, just how do you go about getting the one you want?

Obviously, you should take advantage of the advance seat reservation service, if it is available. If you wait until the day of departure to make your wishes known, your choice of seats may be limited to whatever is left over.

Some airlines publish seating guides to the aircraft they use -- among them, United, American, TWA, Northwest Orient -- and they are available in ticket offices at no charge. These guides can be very helpful in picking desired seat locations, since they illustrate the differing seating configurations in each kind of plane.

For example, TWA's guide has separate seating charts for each of the following aircraft: the new 767, the 727-Standard, 727-Stretch, the wide-bodied 747, the 747SP, the wide-bodied L-1011-Domestic, L-1011-Domestic and International and the MD-80. Other airlines fly the small 737, the DC-8, the DC-10 Standard and the DC-10 International.

When booking a flight, ask what type of plane will be used. By consulting the chart for that plane, you can pick out exactly where you want to sit. You can then ask for a specific seat by row number -- provided it is not already taken.

If you don't have a printed seat guide, you can run into difficulty asking for a seat by row number, since numbering differs on each type of aircraft. For example, on TWA's wide-bodied L-1011, Row 20 is nonsmoking; on its wide-bodied 747, Row 20 is smoking.

Instead, let the reservation clerk know in general terms what kind of seat you want. Among the choices in any search for the perfect seat:

*Smoking/no smoking: Every passenger is offered this choice. Usually, in each cabin nonsmokers sit forward and smokers sit to the rear (though on some wide-bodied planes, smokers and nonsmokers have separate cabins). If you are a nonsmoker easily annoyed by whiffs of tobacco, you will want to ask for a forward seat. But -- a complication -- you don't want to get too far forward, because that puts you within sniffing distance of the smokers in the cabin just ahead. The smoke does drift back when attendants are scurrying through the doorway to serve drinks and dinner.

*Window/aisle: It is the rare air traveler, indeed, who would ask for a middle seat, so the customary choice is between a seat by the window or one on the aisle. Travelers who fly infrequently tend to ask for a window seat, says United's Strickland.

The attractions of a window seat are the view (over some mountain ranges it can be superb); you can rest your head against the window (instead of a stranger's shoulder); and you escape the jostling from the heavy foot traffic up and down the aisle.

If a view is important, ask for a seat forward or well behind the wings for the best sightseeing.

An aisle seat gives you more freedom to stand, stretch and take a short walk; you can get to your briefcase in the overhead compartment; and you don't have to climb over two seatmates to get to the restroom. And chances are you can make a quicker exit from an aisle seat at landing if you are in a hurry to make a meeting or another connection.

The middle seat is the "least desired" seat, acknowledges Strickland. Otherwise scorned, but nevertheless is useful for families or groups who want to sit together.

Forward/rear: For varying reasons, many passengers specify "forward" or "rear" of the cabin, says Susie Schlegel, American Airlines' manager of sales procedures for reservations.

Some travelers believe that seats forward of the engines are quieter; other frequent flyers say they feel less motion at the back of the plane. Airline representatives say most passengers aren't able to notice any difference.

Passengers do notice, however, that it can get smokier farther back in the plane. On the other hand, flyers in the front seats may have to look almost straight up to watch the movie.

You will exit the plane first from a front-of-the-cabin seat. But rear-cabin passengers usually are seated first, so they get first chance at the magazines; they rarely have problems finding an empty overhead compartment to stow their luggage; and they can grab a pillow and blanket before the supply disappears.

*Bulkheads: Airline representatives say bulkhead seats -- those immediately behind the partition separating cabins -- offer the most legroom. Long-legged frequent flyers vie for these seats. Among passengers making advance seat selections, "a great majority," says Schlegel, "want a bulkhead seat because of the leg room." These seats may also be held for families with infant children who need the space for a bassinet.

Another advantage: Because there's no seat in front, you won't have to endure someone reclining his or her seatback almost into your lap.

*Window exits: Most planes have window emergency exits over the wings. Nervous flyers can ask to be seated next to one of them. Disabled passengers or those traveling with children will not be assigned these seats, however, because they might be unable to open the exit if necessary.

*Galley and restrooms: The galley gets "a lot of traffic," says David Aime, manager of Eastern Airlines' Washington ticket office, and the same is true for restrooms. So if you want a nap, ask for a seat away from both.

*Center/sides: This is a choice only on wide-bodied planes, which have two aisles.

The side seats, two or three abreast, are popular because they are next to the windows. If your itinerary takes you over the Alps, you want to see them. But the center section (four or five seats abreast) has much more headroom, because there are no overhead compartments. You can stand up and stretch, an important consideration on long flights.

Also, in the center section, traffic up and down the aisles doesn't block your view of the movie screen. And if the plane's not full (which happens more than you might think), it's wonderful to be able to put up the armrests and stretch out.

*First class/business/coach/charter: This choice depends on your ability, or desire, to pay. On scheduled airlines, first-class passengers, who pay the most for their ticket, get the most room; business-class travelers get less; and coach-class passengers get least.

On charter flights, expect no more legroom than a coach-class traveler and perhaps even less.

Airline representatives point out that passengers can lose their reserved seats if they fail to arrive at the plane by a certain time, usually not later than 10 or 15 minutes before departure. In such case, they will get whatever seat is available.

On occasion, an airline may substitute a different type of aircraft at the last minute, one that has fewer seats or a different seating arrangement. When that happens, seats may have to be reassigned. Usually there is no difficulty, and passengers get approximately the seat they requested.

But sometimes passengers traveling together may be separated, especially if they purchased their tickets individually. For example, two business associates traveling to the same conference may have reserved seats next to each other, but the airline computer shows them only as two single travelers. If seats are reassigned, they may be placed in different rows.

To prevent such a possibility, especially if you planned on some important preconvention consulting, have the airline clerk enter the name of your associate with your reservation.

Contacted airlines say they generally make all seats available to passengers on a first-come basis. An exception, says United's Strickland, are the rows dividing smoking from nonsmoking. Generally they are held in reserve so they can be used in whichever category they are needed.

Some airlines are installing seats designed for handicapped travelers, who, of course, have priority for them.

And, says Eastern's Aime, some seats just are not as comfortable as others. For example, those immediately in front of the galley on some planes do not recline. Such seats are indicated on the airline's computer and are not assigned unless needed.

With many flyers choosing their seats in advance, are you destined to get an undesirable location if you buy a ticket at the last minute? Not necessarily, says Strickland.

All airlines regularly have "no-shows," travelers who have made reservations and then don't appear. The last-minute flyer, getting one of these unclaimed spots, may end up with the best seat on the plane.

Whichever one that is.

NIAGARA POSTSCRIPT: A recent article on Niagara Falls in the Travel section prompted the Corcoran Gallery to note that the Washington museum will display this fall what it calls "the largest and most complete exhibition ever assembled on the theme of Niagara Falls."

Now being shown at The Albright-Knox Art Gallery in Buffalo, N.Y., just south of Niagara, the show moves to the Corcoran Sept. 21 and will remain until Nov. 24.

The centerpiece is the Corcoran's painting, "Niagara Falls," done in 1857 by Frederick Church. Also in the show are paintings, prints, photographs and memorabilia, which, according to the Corcoran, deal with the falls "as the great American icon of the 19th century . . . "