It's a rare businessperson who doesn't know that special meals on commercial flights are more palatable than the infamous potluck ones. But many may not realize that besides the usual vegetarian and kosher alternatives, they can choose all-fruit, all-seafood, low-calorie, Hindu, and Moslem preparations.

That and other tidbits are found in 202 Tips Even the Best Business Travelers May Not Know (Irwin Professional Publishing, 708-206-2700, 1994, $10), by Christopher McGinnis, an Atlanta business-travel consultant. You'd have to be a stay-at-home not to know most of the tips, but even for veterans there are revelations. Here are a few:

· Rather than buying one unrestricted-fare air ticket, you can save 50% if you buy two round-trip tickets that require Saturday-night stays and use only the outbound portion of each.

· If you check your bags at the gate, they're usually waiting by the door as you exit, not at the baggage carousel.

· Most car-rental agencies have more large cars than small ones. By reserving a compact at the cheapest rate, you have a good chance of a free switch to a larger car. If not, at-the-counter upgrades are usually less costly than an advance reservation.

· If you're on a tight budget but expect to grab the tab at an expensive restaurant, take the low-cost lead and order from the health-food side of the menu; your guests will probably follow suit.

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