IN WASHINGTON, it happens every day. People arrive from Hoboken or Timbuctoo knowing they won't be here forever. Washington for them is only a transitive word between Was and Will Be.

Their problem is how to settle in, realizing as they do that these curtains will be too long for the next windows and the sofa will never go up the steps to the hereafter.

It is not only their problem; it is one for most of us. If you postpone living right until you go home, or the children are grown, or you buy a house, or you're elected President or your ship comes in, then you may wake up one day and find out you've missed the boat.

There are only two ways to go about it: Give up and accept the fact that you don't care where or how you live, even if there is pink rose wallpaper. Or settle into each new place as though this, at last, is what you were waiting for.

In the case of the Albert Ndeles, after living in too many furnished quarters, they decided to make a major decorating effort in their rented Washington duplex. Albert Ndele, still a young man though he has already served as governor of the Bank of Zaire and minister of finance of Zaire, is now in Washington while he studies economics at the Johns Hopkins School for Advanced International Studies.

He wanted a home he and his wife could entertain their international friends in, but since he was a student, the cost had to be in keeping. The Ndeles also needed to decorate with the thought in mind of moving once again. They needed practical, movable furniture, yet they wanted an elegant home filled with light and art.

Meda Mladek, a friend of the Ndeles who isn't a professional decorator at all, but an art collector and historian, came to their rescue. When they asked her, after admiring her home, if she would advise them in decorating their apartment, she did better than that. She took on the whole job. Her one condition was that the Ndeles let her treat it as one whole design concept. "You do it your way," Albert Ndele told her, "nothing will be considered too unusual or extravagant so long as the cost is not extravagant."

The Ndeles had rented one of a new group of duplexes on Klingle Place near where Abigail McCarthy now lives. The houses are nothing special architecturally - just imitation Federal - but at least they have sliding glass walls in the first-floor family room. The Ndeles' faces south onto a small garden.

Since the house was rented, there was little to be done architecturally. But Mrs. Mladek had the original off-white or cream repainted a clear undiluted white as a better contrast to the art and to give a clean modern feeling. "I had only specified the living room," she said, "but when Ndele saw it, he thought it looked so good, he told the painter to go ahead and do the whole house." In the living room, Mrs. Mladek had the fancy molding removed (and carefully packed away in the basement to be installed when Ndele leaves). Mrs. Mladek, like architect Hugh Newell Jacobsen, believes that molding stops the eye and clutters a room.

She also believes that one quality, expensive piece of furniture or art in a room, if chosen right, can blend with less expensive furnishings and give the whole room an elegant aura. In the Ndeles' home she has done just that.

In the study, which is organized to the last detail, right down to color-coordinated scrapbooks for Ndele's research files, she placed a handsome African sculpture on a very practical desk. The desk is a floating top on two pedestals which, in another house, can easily expand or contract.

The bookcases were the bargain - $20 at the hardware store. There are of white, hard plastic and serve the purpose while blending into the wall. The television set is white as well, looking a bit like a pop art piece itself on its yellow table. The table can be stacked with mates elsewhere in the house to form bookcases.

At the end of the hall is the family sitting room, on the same level as the garden. Here Mrs. Mladek used white modular Thayer Coggin seating units arranged in a conversational grouping. The stackable plastic tables, here in red, make accent notes against the white. A remarkable painting over the sofa, with attached canvas pieces, is by A. Simotova, a Czech painter.

Through the sliding doors is tha patio. The area is largely "paved" with white gravel, which reflects light not only into the sitting room but also up to the living room.

Upstairs - the piano nobile - are the more formal rooms. The living room, simple light and dramatic, is at the back, overlooking the garden. To simplify the multi-muntined and mullioned window, Mrs. Mladek used vertical blinds (She did the same downstairs). The living room upholstery is one of three black and white patterns that she offered to Ndeles. They chose the one which she thinks has the most African feeling. The tables are lucite, custom-made by Jeffrey Bigelow.

Three tall cactuses in one corner seem almost to be desert sculptures. More plants are used in the corners, between the modular seating units. Mrs. Mladek used dimestore planters, not only to save money, but also for their strong red and yellow shiny plastic look.

The focal point in the dining room, which is in the middle of the house, is the table, a pane of heavy plate glass resting upon acrylic cylinder legs. For china, Wedgewood's green dragon pattern is now on order. The Ndeles' silver was a bargain, a splendid set by Tiffany that was bought at auction at Sloan's.

he vertical blinds seen elsewhere in the house reappear on the dining room windows.

For parties there are five card tables, folding red chairs with matching covers for the tables and lots of bamboo trays, so guests won't have to balance food on their laps. It all worked well indeed at the baby's christening party, to which fifty people came.

On the top floor is a pleasant master bedroom, with an ornate mirror over the bed, another bargain, this one acqired at Thieves' Market.

The children's rooms have Marimekko fabric for curtains.

The whole decorating effort comes across as a place for sophisticated, independent people who don't need the fripperies of colonial-style gold leaf and mahogany to be happy.

And, as a bonus, everything is not only easily portable, but in addition would work together in other ways, other countries. Call it the international portable school of decorating.