B&B. Bed & breakfast establishments are cropping up in every corner of the United States these days. Their reasonable rates, down-home appeal, and savory meals are all part of what is making them so successful. Innkeeping basics

Sarah Milligan can tell you firsthand about the joys and pitfalls of running a bed & breakfast establishment. ``The main theme throughout the B&B scene is that yours is not a commercial establishment,'' she says. ``This is your home and you welcome guests, even though they're paying.''

In the case of Sarah and Jack Milligan, home is Lafayette House, a vintage 1876 Victorian mansion in the historic Lafayette Park area of St. Louis, south of downtown. Despite its antiquity, Mrs. Milligan says, ``Ours is a house, it's not a museum.'' She has some advice for those considering the B&B business:

``It's hard to establish how much money you should bring into the business. It depends on the degree to which you want to operate. You needn't truck in fancy items, if your present furniture will suffice.

``One thing I stress is that there is no guaranteed income.'' The Milligans grossed about $1,000 their first year. In 1986, they grossed about $20,000.

``Do you have good beds? That's vitally important. Do you have enough linens? Towels? Dishes and cutlery?''

``You have to love people, that's the most important thing there is.''

``You have to be very flexible.'' Mrs. Milligan has gotten up as early as 4 a.m. to prepare one of her far-from-skimpy breakfasts for a departing guest.

To be successful in the B&B business, Mrs. Milligan stresses, you have to set rules. In her literature, for example, she requests a 20 percent advance deposit, states that travelers can be picked up on arrival for a small fee, advises that children are welcome but pets are not.

Other factors she cites in operating a Bed & Breakfast include:

``A favorable location, in a good traffic pattern, though you needn't be in the heart of town.''

Zoning that permits your kind of establishment.

Proper record-keeping for reservations and payments.

Keeping your name in front of the public by listing your establishment with tourism bureaus and perhaps with travel agents, and certainly advertising in B&B guides and shelter magazines.

Maintaining a telephone answering device with a current message on it when you're absent.

Despite the demands, you can maintain a family life, Mrs. Milligan insists. ``We entertain. We go out, but if guests need taking care of early in the evening, we go out later.''

``I really love what I'm doing.''