It is vacation time again, and Americans by the millions are rushing to leave hearth, home and the dog to spend two weeks in some place where nobody knows them and probasbly can't understand a word they say.

These people need all the help they can get.

Certainly they won't get it from the professional travel writers and tourist guidebooks, which burble and gush over the scruffiest landscapes, the filthiest beaches, the grimiest cities.

And they won't get it from Aunt Martha or the boys at the office, either. Too bad nobody ever bothered to tell them how simple it all can be, if only they study the four Principles of No-Fault Travel:

* Whatever Your Friends Told You, They Are Wrong.

''Oh, you're going to California?'' your aunt says, ''well let me tell you about a charming little village I stayed in, in 1932. It may not even be on the map, but it's called Nepenthe-by-the-Sea . . .''

Yes, it was indeed charming in 1932, but today Nepenthe is an industrial city, and the industry is the mass production of charm. Everything is charming. Even the gas stations look cute. The motels and billboards start about six miles out of town, and by the time you reach the hundreds of dear little boutiques, either you or your car will probably have had a stroke. There is a public beach, but you wouldn't want to lie down on it. Most of the astonishingly beautiful coastline is fenced off with barbed wire, rude signs and walls with broken glass on top.

Of course, it isn't just Nepenthe that the campers collect in, importing smog and empty Coke cans. Almost every place outside of Cicero, Ill., gets so crowded in the summer that you wonder where all these people can be coming from. The nation's population has doubled since your aunt went on her trip, after all, which means you can't believe anyone old enough to have Memories.

When we went to Athens somebody told us to be sure to look up a fascinating little hole-in-the-wall called the Bouzouki or some such. The real thing, we were assured; it's not even in the phone book, the food is superb and the waiters are characters.

Well, it was in the phone book. And when we got there the line of Americans stretched clear down the block. They were already complaining; it's part of our national style. The prices were worse than Maxim's, the waiters looked cross and the maitre d' was taking reservations for next Tuesday.

* You Can't Read at the Beach.

This was the summer you were going to read ''The Magic Mountain'' for sure. There you are on your towel, propped on your elbows, with the entire Aegean or Pacific or Caribbean laid out before you and the book in hand at long last (at page 11: you can't read on the plane, either). And you start.

The glare is killing your eyes. You put on your dark glasses.

They are smeared with suntan lotion. You stop to wipe them.

Sand patters softly onto the paper. You blow it off.

Out there, a kid on a surfboard almost catches a big wave but falls off. You read page 12.

There is a bump under your towel. You pound it down.

Your glasses slide down your slippery nose. You push them back.

. . . And smear the lenses. You stop to wipe them.

Sand patters softly onto the paper. You blow it off.

A Frisbee game starts up. You read page 12.

A lovely little breeze riffles the pages. You hold them down with both hands.

The kid on the surfboard falls off again. You read page 12. (This is a well-known phenomenon called deja-lu.)

Your elbows hurt. Your glasses slide down. Sand patters softly onto the paper . . .

* Beware of Smiling Cabbies.

Asd we struggled out of the Athens airport, bent down with suitcases and bristling with the shepherd's crooks we had bought in Crete, he was right there, grinning and friendly.

''Oho! Shepherd's crooks!'' he shouted. ''You been to Crete!''

Yes, we puffed, and did he know the Hotel Olympia?

''Oho! Olympia! No problem, no problem!''

He wanted $15 worth of drachmas but let us talk him down to $12 because he came from Crete himself. All the way in he chattered cheerfully about Crete and his family and the guide business he ran on the side and his cousin in Cleveland. From time to time he turned to grin at us, eyes twinkling with quick intelligence. He pointed out some ancient sites as he weaved so gracefully through the gnashing trafffic you'd think we were invisible.

And when we got there he opened the door for us with a flourish and set our bags on the sidewalk. ''No problem, no problem!'' he said.

Oh such a delightful man.

We tipped him the rest of the $15.

When we left the hotel a week later to return to the airport we got a cab from the nearby rank. The driver scowled at us. How much? we said. Four dollars, he said. The standard charge.

He didn't say another word all the way out.

Neither did we.

* Avoid Jolly Groups Unless You Are in One.

A friend of ours refuses to fly on any plane where more than five people know each other. He has had it up to here, he says, with strangers hanging over the back of his seat, dropping cigarette ash on his head while they chat with his seatmate.

It is worse in foreign countries, because Americans talk louder in foreign countries. This means you have to listen to what they say and can no longer concentrate on ''The Magic Mountain.''

They talk about where they have been, their adventures with the local money, the plumbing, their gewgaws and the number of times they ordered martinis and got Martini vermouth. Then they start to talk about San Francisco. Americans abroad always talk about San Francisco.

Once we stopped at a motel in Elyria, Ohio, and found the pool full to the gunwales with burly young policemen wearing identical Burt Reynolds mustaches. Wives lounged shyly on the coping, and small children hurled themselves into the water every few seconds. The men roared at each other continuously.

After dinner they disappeared into their rooms, but toward midnight one couple went for a moonlight dip. Another couple joioned them, and another, and before you knew it they were all out there again, roaring and hurling. The manager eventually got them to quit for the day.

They were still feeling jolly when they went back to their rooms, so a soccer game developed in the corridor outside our room. A lost child started to cry. There seemed to be a water fight.

We all got to sleep about 2.

Recollected in tranquility, it was kind of a fun evening at that. All that clean-cut merriment. If I ever run into an Ohio cop on a plane trip, I'll lean over his seat-back, spill some Tab down his companion's neck and ask him if by any chance he attended a convention in Elyria one summer.

Then we'll talk about San Francisco.