It was summer which produced the adage about never keeping fish or guests more than three days. In the warm months of the year, when everyone wants to be where they are not, guest rooms fill up as fast as an unwatched bathtub.

All visitors, of course, do what mother always said: Come bearing gifts, always say please and thank you, make your own bed unless there is a maid standing by, and, later, write a bread-and-butter letter saying it was the very best time you ever had. But there is more to being a glittering guest or a heavely host.

For the host:

Unless you plan to link up like a Siamese twin, have extra keys made for houseguests, the colored plastic ones so people will remember red key for the back door, blue key for the front.

If this a not the return of the native, provide maps. The Map Store, 1636 Eye St. NW, has a wide variety, including "Washington, D.C. and Vicinity," for $5.95. This one is particularly good for downtown, as it has the subway routes clearly indicated. A "New Street Map of Washington D.C., with Alexandria, Arlington and parts of Montgomery and Prince George's County" ($2) is helpful for guest with a car.

And a "Guide to the Nation's Capitol and the Smithsonian Institution," for 80 cents, is a tourist's treasure, giving monument locations and hours and unraveling that tangled skein of Smithsonian buildings.

Ask your guests if they will be there for dinner. It will make your life easier, and also lets them know their presence is desired, but not required. If they are only going to be in town for a day or two, ask if there is anyone they would like you to invite over for a drink or dinner.

One frequent houseguest praises any host who provides a bedside lamp and perhaps a book or two. And towels -- big enough to dry with, not the skimpy kind that lie in the back of the linen closet until a guest shows up. Beside bug spray is the suggestion of one visitor who has too often been victimized by The Midnight Mosquito. Who dares to awaken a host at 3 a.m. with a request for a fly swatter?

If you'd like a Sunday of silence, put a copy of the Sunday papers outside each bedroom door. You may not see your guests till sundown.

For the houseguest:

Let your host know what your plans are, particularly when you're arriving and when you're leaving. Too many people announce at 4 p.m. that their plane leaves at 5.

Help with the dishes, and everything else, and don't expect to be waited on.

For room service go to the Hilton.

Don't walk off with your host's belongings, the book you didn't quite finish, the suntan lotion you took to the beach. And don't walk off without your own. The belt you leave behind means a trip to the post office.

Check with the cook before making dinner donations. Dumping three dozen steamer clams on a person who has spent hours preparing something else is not the act of a friend.