One adventure-seeking Washington man prepares for his summer vacation by packing his suitcase with bright new bikini underwear.

For other vacationers -- with a more complete wardrobe in mind -- the urge is to jettison last year's faded tennis shorts and swim suit for trendy new designer garb.

Thousands, if not millions, of us struggle to lose a few pounds before parading our year-older bodies on the beach.

And now is not too early to begin planning your summer lark: Popular spots tend to book up early. Besides, dreams of pleasures ahead can brighten winter-worn psyches.

"Play," says psychotherapist/vacation expert Stephen A. Shapiro, "is serious business."

An authority on getting you mentally tuned to better enjoy your vacation, Shapiro considers advance preparations part of our anticipation of a good time -- and that, he says, is healthy.

"It's good that people have these expectations. They tool up -- just like planning a party and cleaning up the house in advance. You get rid of a lot of delayed tasks you've wanted to do.

"You diet, you think about clothes. You don't want to go on vacation without new stuff. You want to be fresh."

If you're among those work addicts who find it hard to make a break with the desk, consider this:

We need regular vacations, Shapiro says, "because we live in a high-stress world, which makes very heavy demands on us. They help reduce the stress." Unrelieved, the tensions of the job can bring on "headaches, insomnia, irritability, family conflict, alcoholism."

Holidays also "get us outside our normal role, where we "tend to get frozen." A vacationer who takes up "scuba-diving, meditating or dancing," he suggests, has a chance to expand his or her capabilities. The shy guy who only takes orders in the office may become the trail-blazing leader of a back-packing expedition.

"We realize we're richer people than what our daily lives offer."

Too often, he contends, modern Americans "work long hours because overtime means more money and more purchasing power and, therefore, more possessions." Then they postpone the long-dreamed-of jaunt abroad, spending their days off "painting the house, cutting the grass, sealing the boat and performing all the other upkeep functions required to maintain their possessions in working order."

But sometimes we expect too much.

"A vacation can give you a change of pace, a different perspective, a tune-up," says Shapiro, co-author of Time Off: A Psychological Guide to Vacations (Anchor Press, 102 pages, $5.95). "But it's not going to change the structure of your life. If you're unhappy with your life," when the sun sinks in the West, "you still have to return to that life."

Some people set out on a trip "expecting perfection -- that everything is going to go smoothly, that it will be Cloud 9 all the way." They can put up with the daily hassles at home and office, but they want two trouble-free weeks at a resort in return. Then the plane is delayed, the luggage lost, the hotel overbooked and their stomach begins to churn ominously.

What these travelers forget, says Shapiro, is that "Vacations occur on earth, where nothing is perfect." The result is that their "overidealized expectations can lead to a letdown." And they come home gloomy. "It's terribly depressing to go on vacation and not enjoy yourself."

A good time not to take a big vacation, he says, is when "you are overstressed on the job." Try and work out what's troubling you before you leave because it's going to be there when you get back. Or in "a grief situation, you may feel you have to get away all by yourself. That's a disastrous idea. After loss, we need familiarity and friendship."

By taking a long look at ourselves before setting out on a holiday -- who we are, and what we like and don't like -- says Shapiro, a vacation consultant to American Express and other corporations, many potential vacation disappointments can be avoided. As director of Volunteer Counseling Service of Rockland County, N.Y., he has advised hundreds of clients on the emotional problems that can chill those all-too-short weeks away from the job.

When vacation time approaches, many people, he has found, experience a variety of fears that threaten their good times. Some are afraid "of not being able to have fun. Sometimes it's unrealized, but sometimes it's justified. They shut down their pleasure factors."

Others feel "an unrecognized fear of going away and leaving their children or a fear of doing something new, something they don't know how to do. If they're fearful, and they don't know it, they tend to get angry and indifferent.

"A receptive, emotionally healthy vacationer," he writes, "can find a rewarding and fully satisfying experience at home or virtually any place else, while another, trapped by inappropriate emotions or by poor preparations, can be miserable even in the best surroundings."

Shapiro divides holiday planners into two categories: the "impulsive" vacationer, "who merely glances up at a travel poster" before making a decision, and the "plodder," who pores over travel folders for months. Pleasure and disappointment can await both.

If you're impulsive, you may get satisfaction out of the spontaneity, but many people select holidays on the spur of the moment "that are poorly suited to their real needs." Advises Shapiro: If you're the kind of person who can roll with the punches, "you can afford to be impulsive." Otherwise, consider being more deliberate about your planning.

The plodder avoids obstacles but often loses the excitement of the unknown. "Think, 'This is what I did last year and I loved it' or 'It bored me.' The importance of knowing personality type before making vacation commitments cannot be overstressed."

Other considerations:

If you're clock-conscious, would you be happy with a loosely structured holiday? Maybe, but only for a change of pace. Or are you always late for appointments? That might cause havoc on a tightly scheduled tour.

Share expectations with your travel partner. Shapiro describes a couple vacationing in Hawaii who didn't, and their trip was shattered. The wife dreamed of romance; her husband pursued only a love for anthropology.

Maybe you and your mate have been invited by an office colleague and spouse for a week at their beach house. It's a cheap vacation, says Shapiro, but it will inevitably turn out to be a week of shop talk. Perhaps you would prefer to have the time along together -- as far from office politics as you can get -- even if you have to pay for a hotel room.

Are you bored with visiting relatives each year? On talk shows where he's appeared, Shapiro says, people phone in complaining about being "in a rut. They load up the car and always go to the relatives." There's nothing wrong with seeing Mom and Dad, but those who do it repeatedly "tend not to plan. No decisions have to be made. They are restricting themselves."

For himself, Shapiro likes to combine as many kinds of vacations as he can.

"If I go to Egypt, I'll want to mix resting around the pool with some archeology. I wouldn't want to go on a vacation and spend all the time in the pool, especially if a pyramid was 10 miles away."

Length of vacation, he says, depends on particular circumstances. "A super-tense person may need two or three days to unwind at each end.I generally need 10 days to two weeks away. It's hard to step down out of the pressure."

And returning to the work world? "I come back to the home environment a day or so early and do something nice -- see friends, go to the movies." If you don't ease back, you may face re-entry problems.

"One day you're swimming in the pool balancing a daiquiri on your belly," and zap, the next you're back at the desk in a daze looking at an "18-hour" stretch ahead.