Business travelers know jet lag can be more than a case of the blahs; it can be a real detriment to business dealings. But many frequent fliers have developed ways to combat travel fatigue: They take advantage of airport lounges; they know which carriers provide the best seats and which seats have the least noise and vibration; they order special meals at no extra cost; and they even perform exercises in their seats to stimulate circulation and prevent stiffness.

Airport clubs

Airline clubs provide travelers with places to relax, entertain, or carry on business at the airport. Before you join a club, however, make certain it has clubrooms in the cities where you frequently fly.

In the accompanying chart, you can compare the current membership rates and services of the eight major clubs. In addition to membership prices, the chart indicates which clubs offer free liquor, complimentary cards for spouses, local phone and message service, and other extras.

Two airlines also provide free services you may want to take advantage of: Delta's Crown Rooms are small waiting rooms, located in 38 airports, which you can reserve free of charge when you fly Delta. The rooms, which offer free liquor, must be reserved at least 24 hours in advance through a local Delta sales office. Pan Am's Frequent Traveler ID card often permits special check-in privileges, more personal service from ground and air personnel, and the cashing of personal checks up to $100. You also get a quarterly newsletter with travel tips. To request membership, write Frequent Traveler System, Pan American Airways, P.O. Box 2212, Boston, MA 02107.

Business class: No big deal

The airlines are spending plenty promoting so-called "business" classes. But when you get right down to the bottom line -- that is, how comfortable your posterior is -- you may be in for a disappointment. The extras often include a free in-flight stereo headset, a free movie, a couple of drinks, but it's still the same old sardine can. Before you pay top business-class dollar, inquire if the flight is uncrowded enough to reserve a couple of empty seats next to each other in coach.

Not all the news about special accommodations is bad: Comfortable seating arrangements in business classes are found on many of the wide-bodied airplanes. Instead of the standard 10 seats across, some airlines -- including TWA, Pan Am, British Airways, Northwest, Olympic, Sabena, and Swissair -- have reduced the number of seats to between 6 and 9. When you make your reservations, ask about the seating layout and watch for such good deals as Pan Am's New York-to-California 747s with 8-across seating and free drinks at full coach prices.

If you arrive at the ticket counter early, ask to see a seating chart. A glance at the airplane's floor plan will reveal where there's extra leg space. Most seasoned travelers stay away from the galley and restroom areas where insomniacs, as well as stewardess hustlers, tend to congregate -- usually on your armrest. Nonsmokers should, whenever possible, select a seat at least five rows in front of the smoking section.

Eating and sleeping

Most doctors recommend that travelers stick to light, high-protein meals that have less of the sugar, salt, and spices that lower the blood sugar and make you groggy. Most airlines offer a variety of special meals -- kosher, low-sodium, low-calorie, low-carbohydrate, vegetarian. Any of these meals can be ordered at no extra cost when you make your reservation.

Unfortunately, there's no way to completely eliminate discomfort when you have to reset your sleep schedule after changing time zones. When you can't afford a rest day, sleep scientists at the Sleep/Wake Disorders Center at the Montefiore Hospital and Medical Center in the Bronx, N.Y., say it's usually best to take an active approach. One study indicates it takes longer for a traveler to adapt if he isolates himself in his hotel room than if he immediately becomes part of the local scene. Napping often does more harm than good when you're trying to adjust quickly to a new time zone. Sleeping pills aren't recommended either, since they can cause side effects and, in certain cases, linger in the body for several days.

Exercise prevents stiffness

Stiffness and grogginess after a long flight are, in large part, the results of poor circulation. A number of in-the-chair exercises were designed by Folke Massfeldt, a Swedish physical therapist and fitness adviser to Scandinavian Airlines (SAS). The exercises have even been used by NASA astronauts to relieve space-sitting weariness. Here are some examples, from SAS In-the-Chair Exercise Book (available by sending a bank money order for $4.95 to SAS, Suite 1465, 630 Fifth Ave., New York, NY 10111). Incidentally, these exercises work as well when performed in the privacy of your office:

1. Jogging on the spot (a warming-up exercise). "Jog on the spot" by raising your heels alternately as high as possible. At the same time raise your arms in a bent position and rock the torso rhythmically forward and back as you do when walking. Continue one to three minutes.

2. Raising on the toes (improves blood circulation to the legs). Sit with elbows on knees, bending forward with your weight pressed down on the knees. Lift up on toes with the heels as high as possible. Drop heels and lift toes. Repeat exercise 30 times.

3. Shoulder rolling (Stimulates the joints, relaxes shoulder muscles). Smooth rhythmic movements "lubricate" the inner joint. Move the shoulders gently and rhythmically, at intervals, describing large circles in both forward and backward directions.

4.Head turning and nodding (stimulates joint capsules and cartilage in upper spinal column). Occasionally, do the following: Turn the head the fullest extent to the right. Nod a few times. Do the same toward the left. Repeat the exercise six times.

5. Forward bends with stomach in (stimulates bowels and blood circulation). Draw the stomach fully in. Drop the trunk forward while lifting the front of the feet high up. Place the toes back on the floor, relax the stomach muscles, and raise the body upright again. Repeat about 30 times.

6. Hand turning (stimulates the wrists). The cartilage and joint capsules in the wrists also need stimulation. A good way to achieve this is to turn the hands all the way over and spread the fingers. Return hands to original position and relax them. Repeat 15 times.

7. Foot rolling (stimulates the ankles). Exercise the ankle joints now and again by rolling the feet in large circles to the full extent of their movement. Repeat 15 times in each direction.

8. Knees up against the elbows (speeds up blood circulation). Now and then, at regular intervals, one should increase the blood circulation by setting to work large groups of muscles. This is one way: Drive the left and right knees alternately up toward the opposite elbow. Repeat 15 times in each direction.

Sources: OAG/Frequent Flyer magazine, published by Official Airline Guides Inc., New York, N.Y.; The Complete Handbook for Travelers by Hal Gieseking, published by Pocket Books, New York, N.Y., (c) 1979; $7.95; Economy Traveler newsletter, Box 547, Menlo Park, CA 94025; subscription $10 annually.