If you're going abroad and want to be certain you stay healthy, it's mostly up to you. It takes only a little common sense and vigilance to roam illness-free in most of the places Americans are likely to go. But remember, you must take certain precautions.

One of the most common mistakes made by travelers on long jet trips is to "party it up" before leaving and sometimes to continue the celebration along the way. That's a good way to ruin a couple of precious vaction days abroad. The reason, of course, is that drinking compounds the effects of jet lag, the fatigue, insomnia, anxiety and sometimes depression that often hit the jet traveler because his "body clock" is still set on departure time while the destination may be hours slower or faster.

Big Ben may say it's noon when you arrive in London, but your brain and everything it controls is reading 4 a.m.

Here are some suggestions on how you may reduce jet-lag symptoms:

For short flights, eat, go to bed and arise the first day according to home time.

When traveling west to east, go to bed progressively earlier before the trip (vice versa east to west).

Use food and drink sparingly during the flight and the first three days after arrival.

On long flights, try to plan a halfway stopover.

Arrange your schedule to shorten the day rather than the night.

Don't forget that it's not always convenient to buy first-aid items abroad. Consider taking along insect repellent, sleeping pills, water purifying tablets or Colorax or iodine and an eyedropper if your journey is to the tropics. And don't forget bandages, antiseptic and aspirin.

It's also a good idea to carry a spare pair of eyeglasses, as well as a supply of all prescription drugs you mat be taking. And be sure to carry a letter from you doctor saying he has prescribed the drugs for you. This can avoid embarrising problems with customs officers who may see your supply of pills.

Remember that all drugs do not have the same trade names in each country. A trade-name drug that is a blood-thinner in one country may be the name of a drug to treat epilepsy in another. This information is especially valuable if you happen to be visisting a country where many drugs that are obtainable only by prescription in the United States can be bought over the counter.

If you are considering drugs to prevent diarrhea, be aware that public health authorities advise against taking antibiotics prophylactically, even though some of the antibiotics do reduce the incidence of that illness. One reason is that this kind of usage worsens the growing problem of bacteria that become resistant to antibiotics. Those antibiotics are than useless if the occasion arises to fight an infection caused by this particular bacteria.

Another reason is that they can cause side effects in the intestines that can be as unpleasant as traveler's diarrhea.

There is considerable current interest in a nonatibiotic drug called subsalcylate bismuth in preventing or reducing the severity of diarrhea. It has not been recommended for routine use, but tests are under way at the University of Texas Medical School in Houston.

Much has been said and written about avoiding nonpeelable raw fruits and vegetables in tropical and undeveloped countries, as well as raw freshwater fish, rich or creamy sauces, unchlorinated drinking water and ice and unpasteurized milk and cheese.

These are the primary sources of the bacteria and other organisms that can extract a high price for a night on the town. Remember that the water condensation on the outside of cans or bottles might have been contaminated by the ice in which it was packed. Even though the contents are safe, the droplets on the outside may not be. Take a few seconds to wipe the container dry before opening and pouring.

According to public health representatives, tap water that is uncomfortably hot to the touch usually is safe for bursing your teeth after cooling at room temperature in a clean container.

If pure water for drinking is unavailable, or if you're unsure of its purity, drink beer, wine of soft drinks or purify you own water with Halazone of Globaline tablets. Or boil the water for 10 minutes or else add 10 dorps of Clorox or five drops of iodine to each quart.

In case you get diarrhea, here is an effective antidote that is helpful in minimizing the symptoms: In an 8-ounce glass of orange or apple juice (rich in potassium) add 1/2-teaspoon of honey, corn syrup or sugar and one pinch of salt.

In a second glass mix 8 ounces of carbonated or boiled water with 1/4 teaspoon of baking soda. Drink the first glass and follow it with the second glass. Repeat after each bowel movement.

If you or a traveling compainion has a medical problem, it may be useful to be aware of the services offered by most commercial carriers. For example, they carry a supply of first-aid oxygen and a masks in addition to the automatic emergency oxygen referred to by flight attendants during the pre-takeoff briefing. One use is for passengers with respiratory or circulatory problems that might worsen aloft when cabin pressure is maintained at the level of 5,000 to 7,000 feet.

Because the retina of the eye has a high oxygen demand, it is recommended that persons suffering from retinal disease receive supplemental oxygen at cabin altitudes above 6,000 feet, according to Dr. George J. Kidera, medial consultant the United Airlines.

While all U.S. airlines carry firstaid oxygen, you must make advance arrangements if your need is for the more elaborate supplemental oxygen equipment required for patients with retinal or other problems.

Less serious, perhaps, but annoying and painful are pressure changes that effect passengers with an upper respirator infection or allergy as well as other problems that can cause the lining of the notrils to swell.

As the plane ascends, the air in the sinuses expands but it can't escape through the swollen nasal passages. When the plane descends, the sinus air contracts and must be relieved by outside air that has difficulty getting through the nose. The consequenct is pain in the area of the sinuses.

Dr. Kidera recommends antihistamines and decongestant nasal sprays before takeoff to prevent these problems.

A similar problem arises when the eustachian tube that connects the ear with the trachea is obsturcted. The pain occurs on descent when the increasing air pressure causes air to force its way through the tube into the ear to equalize the lower pressure there.

A nasal decongestant 30 to 45 minutes before descent can prevent this problem. Also, it's a good idea to avoid sleeping while the plane descends. If necessary, pinch your nose shut with your fingers and swallow several times. This forces the air through the tubes and into the ear chambers, equalizing the pressure and relieving the pain.

If the congestion is so bad that none of these techniquies works, it's a good idea not to fly until it clears up.

Hopefully, before you even bought your ticket you checked with your physician to make sure the immuinizations normally called for even in the United States are up to date. The most importatnt of these are polio and tetanus for all ages, as well as measles, diphtheria and whooping cough for youngsters.

For certain areas of the tropics or in undeveloped countries, immunizations for the prevention of hepatitis a, yellow fever and typhoid are adviseable or even required for entry into a nation.

According to The Medical Letter, a nonprofit publication that evaluates drugs and therapeutics for physicians, malaria is probably the most prevalent disease in certain countries. Protection is highly important. Destinations such as Africa, India, Thailand, Yeman, Turkey, Nicaragua, Brazil and Colombia are among those countries with malaria problems, particularly outside large cities.

Anit-malarial drugs shoud be taken weekly beginning one week before arrival and continuing for six weeks after leaving the area.

Although the World Health Organization says there is no longer a danger of contracting smallpox anywhere in the world, some countries still require evidence of smallpox vaccination as a condition of entry.

Some countries also require cholera shots for entry despite the conviction of some public health authorities that cholera immunization is of minimal benefit.

For the vast majority of travelers, remember to get plenty of rest before leaving on your trip, watch what you eat and drink, make sure your vaccinations are up to date and take along a few aspirin, just in case.