One thing Washington never is in short supply of: houseguests. They come from all over -- to promote a cause, lobby a senator, get an amendment tacked onto a bill or simply frolic around the Tidal Basin. Spare rooms tend to fill up faster than a reservoir in a rain storm and if guests have a season, it is summer.

To keep all that familiarity from breeding contempt, following are some rules for being a good host and a good guest. First, the guests:

Realize that though you may be on vacation, your host usually is not. Don't expect the people you're staying with to take on the role of nanny -- making your phone calls, driving you from place to place, herding you through museums, settling in for three-hour lunches. Arm yourself with a house key, a map and the number of a taxi company and set out to see the city by yourself.

If the household has help, leave a tip, especially if you've been given special attention -- clothes ironed, shoes polished, etc. One embassy wife says that the only thing that irritates her more than houseguests who totally ignore the maid are the ones who slide out to the kitchen to "make friends." This always involves asking the maid personal questions, she says, and, "I hate it when a guest comes bouncing into the living room to announce, 'Did you know Maria is having trouble with her husband?' First of all, it implies that I never talk to Maria, and secondly it's an invasion of her privacy. The same guest would be appalled if Maria were to walk into her bedroom and ask about her life."

Another way to show appreciation is to leave a small gift in the center of your unmade bed when you leave. If there is no maid, ask your hostess if she'd like you to change the sheets.

Find out when meals will be served and get there on time. If you've made other plans, give plenty of notice. There's nothing more irritating than holding dinner for people who stroll in an hour late and explain that they grabbed a bite on their way home to "save you the trouble."

If you are making long-distance phone calls, get the charges from the operator when you make them, keep a list and, at the end of your stay, tote it up, add the appropriate tax, and leave a check for your host. Don't force someone into the embarrassment of having to write you, asking you to pay up.

If you're sending children off to stay with someone, make it clear that they'd better be twice as good as they are at home. That means making the bed without being told, volunteering to help with meals, clearing the table, remembering that towels go on the towel rack, and that if food that they don't care for is served at a meal, "yuck" is not a proper response.

Check drawers and closets before you leave. Anything you forget will mean a trip to the post office for your host. And finding a box to put it in, and mailing tape, and all the other nuisances involved with sending things by mail.

Even though you shouldn't wrap yourself around your host, guests who go to the other extreme and totally disappear are being rude. Don't make so many plans to see other people and do other things that your host feels that you view him not as a friend but as a hotel.

Hosts, in their turn, can make a guest's stay more pleasant by small, personal attentions, such as leaving a bottle of mineral water and a glass on the bedside table. Waking in the middle of the night wanting a drink of water and having to make your way to a strange bathroom to get it can be troublesome. It's also nice when hosts leave a book or two next to the bed. Of course, most people bring something to read, but other people's books are always so much more interesting.

When your guests arrive, let them know any events you've scheduled for them -- dinner parties, theater, etc. Then they can plan their other activities around those. And if you are giving a dinner, ask if they have friends in the city they would like you to include.

Keep a batch of extra housekeys on hand. People often forget to return them. Maps, too. And leave a local telephone book in the guest room. The gestures will be appreciated.

If you're doing a laundry, ask your guests if they have anything that needs to be washed.

Everyone remembers to put out towels for the houseguest, but not everyone provides a place to hang them. If it's a shared bath, make sure there is a free towel rack and counter space for a shaving or makeup kit. One woman who didn't discovered that her guest had hung his wet towel and washcloth over the back of a chair in his bedroom; the finish was ruined.

Warn guests of any house rules against smoking. Some would rather stay in a hotel than curtail their habit.

And if you'd like to be really nice, particularly if your houseguests are from out of the country, lay in a supply of stamps. Postcards are easy to find; stamps are not.