DON'T TRY TO catch the Metroliner on Saturday -- it won't be running . . . Carry extra fuses for the car's air conditioner . . . Remember that airline flight schedules are still curtailed.

Keeping these and other tips in mind, along with careful planning, should make it easier to get a bang out of that Fourth of July trip -- or any hectic holiday weekend.

Whether travelers drive, fly, take a train or board a bus, they should always allow extra time, relax, and avoid a short fuse.

The American Automobile Association expects 6 or 7 percent more traffic over this July 4th than last year, and advises motorists to "plan the trip so when you get out on the highway you're not reading a map and becoming part of the traffic problem." On major travel weekends, the AAA says, drivers need more time because of "possible backups at turnpike booths, roadway access points, and even at some restaurants."

Since the weather probably will be hot, be sure to check fan belts, cooling systems, batteries, and tires ahead of time -- and take replacement fuses for the air conditioner. Sometimes it's better to travel earlier in the day, the AAA notes. It's cooler and you have "better options in selecting overnight accommodations." Again, when you're ready to go back home, "allow enough time from a safety standpoint as well as for comfort" -- returning travelers may be tired and sunburned.

The Air Transport Association says airline passengers should phone for reservations in "off-peak hours," at night or early morning. "Early reservations are recommended more than ever."

Not only are flight cutbacks that originated with the air traffic controllers strike "still in effect," the ATA says, but "people are waiting longer to reserve" as they seek the cheapest fares. "There are more problems than ever on holidays."

Use public transportation to the airport "to help relieve traffic congestion, and get there well in advance to allow time for baggage check-in and security inspection," the ATA advises. Never put cash, jewelry, medicines or important business papers in baggage to be checked; carry those and other valuables with you. Try to avoid the period when airports are busiest; the best time usually is around midday or late evening. And "be prepared with alternative flights or dates in case your first choice is already fully booked," ATA suggests.

Amtrak points out that the majority of its seats in the Northeast Corridor -- all trains from Washington to New York and Boston -- and on the Colonial to Richmond and Williamsburg, are unreserved (exceptions are the club cars and the Metroliner).

For weekend holiday travel, Amtrak recommends buying tickets in advance at city ticket offices or from travel agents to bypass possible long lines at train station ticket windows. There should be no problem getting aboard a train to Philadelphia or New York, Amtrak says, but it could be standing room only on the return trip from Manhattan. Occasional train riders who prefer the Metroliner should be aware that Saturday service was discontinued last fall.

Trailways Inc., has found that it's becoming the trend for holiday weekends for most of their riders to be "on routes 500 miles or less -- a one-day trip." And they're not worried about handling any overflow.

"Unlike the airlines, we don't take reservations. We recommend people call in to get schedule information, then plan on getting to the bus station with 45 minutes to spare so if there's a large crowd at the counter you'll have time to get a ticket and put luggage on board."

Trailways [and Greyhound] "guarantees you'll get on -- if there's not enough room we'll roll out another bus." But passengers must be at the station on time.

Looking for a colorful, tasty, non-explosive July 4th destination where only your diet will be in danger?

Kutztown, Pa., will hold its 33rd annual Pennsylvania Dutch Folk Festival for nine days beginning Saturday, July 3, sponsored by the Pennsylvania Folklife Society. As the good folk put it, it's "koomin fescht" (coming fast) and in "nay treit om oll" (no time at all) they'll again be demonstrating their craft skills, portraying their ancestors' lives, making music, and serving their famed-and-fattening dishes like funnel cakes, shoofly pies and schnitz un knepp. "Vos a vay to schpend a day!" (Native Pennsylvania Dutch still speak a dialect, with variations, that was brought there by early German settlers beginning in 1683.)

Though some Mennonites will be on the grounds, which cover 35 acres, the strict Amish do not take part, explains a festival spokesperson. The "more worldly" and more numerous Pennsylvania Dutch will "impersonate the Amish." Accommodations in private homes will be available if you want to spend the night. Just ask the women's club members in the hospitality tent.

Kutztown is situated between Allentown and Reading. Take I-95 to Baltimore; then Rtes. 495 to 83N to 30E to 222N, all of which are dual-lane highways (222N becomes one-lane each way just outside of Reading and continues for about 20 miles directly to Kutztown). Figure on a four-hour drive. Admission: $5 for adults, $3 for children under 12, which includes everything except food, souvenirs, or accommodations.

A few other last-minute holiday suggestions:

* In Boston, the Boston Pops Orchestra will play on the Esplanade on July 4, fireworks will erupt over the harbor at 10 p.m. on the 4th and 5th, and the premiere performance of a play about historic Old Ironsides will be given adjacent to the vessel on the 5th.

* New York City will present "Harbor Festival '82" on July 3-5, with open houses aboard NATO warships at the Passenger Ship Terminal, arts festivals at Battery Park and in lower Manhattan, and other activities.

* Fredericksburg, Va., will hold its Fredericksburg Heritage Festival for three days, beginning July 3, to celebrate the city's colonial, Revolutionary and Civil War past with music, games, a river raft race, food and fireworks. A re-enactment of the Battle of Fredericksburg is scheduled for 4 p.m. on July 3.Confirm schedule of events by calling (703) 373-1176.

The World Health Organization has begun worldwide distribution of the April edition of its magazine, "World Health," devoted to the subject of "Travel and Health." It may give nightmares to some travel agents and squeamish tourists, and it's definitely not recommended reading for hypochondriacs.

On the other hand, as the magazine points out, " . . . whether he or she travels on business, on holiday or for family reasons . . . travelers should ensure that all precautions are taken against any preventable disease by immunization, drugs and careful behavior." Included inside the publication is a small, full-color booklet, "Be a healthy traveler," intended as a "basic guide." Dangers covered range from casual sharks to casual sex.

For a free copy of the magazine, write to the Pan American Health Organization, 525 23d St. NW, Washington, D.C. 20037, and mention the month and title. Allow for a possible brief delay in mailing -- the magazine is bulk-shipped to Washington from WHO's headquarters in Geneva.