PICTURE THIS: You've just managed to get the car loaded with the children, the suitcases, the tennis rackets, the rubber rafts and even some food for the road as you head toward two weeks at the beach. Just as you hit the highway, there's a wail from the 5-year-old in the back seat. "Where's my teddy bear?" The answer: It's in the suitcase at the bottom of the pile in the trunk. Naturally.

Or what about this. You've just headed away from an endless winter, and flown to Puerto Rico. You get there still in your woolies only to discover that your baggage has been mislaid, and won't arrive for a day or two. And somehow you've filled your carry-on bag with so many business papers that there wasn't room for a change of clothes or even your toiletries. Naturally.

Packing--it can be the most important part of planning a trip. A wide-open empty suitcase can be a daunting challenge, but there are ways to make vacation stuffing relatively painless. After many mishaps, here are some cautionary notes from the Fearless Traveler:

* Match your luggage to your mode of travel and to the duration of your trip. Unless you are traveling by boat, it is unlikely that there will always be someone to tote your luggage. Try not to take more than you can manage yourself. Recent trends in luggage that stress the softer, lighter bag make it possible to carry those bags yourself. Andre Spearman, assistant manager at Mark Cross in Georgetown Park, agrees. "All of our luggage is soft. What we seem to do the most business with is the carry-ons and garment bags."

* Consider trying to bypass luggage lines by cutting down and carrying everything aboard in an under the seat bag and a garment bag. This, of course, depends on how long you will be traveling, but remember, for a man, a garment bag should be able to hold two suits, several shirts and two pairs of slacks. A carry-on or weekender can manage shoes, underwear and toiletries. A woman can pack dresses, skirts and trousers in her garment bag. Blouses or shirts (particularly when wrapped in plastic bags) can fit in a carry-on or small suitcase, along with toiletries, shoes, lingerie and other essentials.

* Never travel without carrying your nightwear, toiletries and a change of clothes (particularly if you are entering another climate) along with you.

* Avoid inevitable disaster by remembering what not to put in your luggage. Don't put jewelry or valuables in bags you are checking. Don't pack wet clothes unless they are encased in plastic. Don't carry glass bottles. Just remember it doesn't matter if you are heading off to a rustic cabin in the woods or to a first-class hotel--it takes time to get spilled shampoo off an angora sweater or a silk tie.

* Think of the convertible pack. Several luggage companies make this new bag, particularly favored by college-age travelers, which can be used as a suitcase with a shoulder strap, or as a backpack. It is usually made out of water-resistant nylon and may have a concealed suspension system. Available in varying sizes, the bag may be small enough to fit under a plane seat or large enough to get you through a two-week journey.

* Plan your packing. It sounds elementary, but many people do not really consider whether they will want or need everything they pack or, for that matter, where things are. Luggage and handbag designer Barbara Bolan offers her plan: She charts out exactly what she intends to wear. For a 10-day trip, she packs five daytime outfits with two extra blouses or tops. She knows exactly what is in which suitcase. When she prepares for a long trip, she bags accessories together.

* When traveling on the road, pack the family car with convenience and safety in mind. In an age of compact and subcompact cars, it is clear that how you pack the family car for nearby or faraway trips can be more important than ever before. A family agreement, made ahead-of-time, about how much each family member will be permitted can save a last-minute frenzy. Make it clear that extra baggage will not replace the auto tool kit and safety equipment. When loading the trunk, it is a good idea to pack less-frequently used articles on the bottom, followed by bags and equipment needed daily and then top off with garment bags. A car-top carrier should be avoided if possible because it creates more wind resistance and therefore uses more fuel. Never put loose objects on the dashboard or rear shelf--they obstruct vision and can become lethal objects in case of a sudden stop.

* Don't take more than you need. And remember, you need less than you think.