Until last week, when the Supreme Court ruled that lawyers could advertise their services, most people would not have thought of hiring a lawyer unless they needed one. Where would American industry be today if it had operated on that principle? Just sitting around, waiting for people to discover that they needed designer sneakers?

To be sure, the new guidelines are modest ones, in keeping with the dignity that the profession tries to maintain on its off hours. But this could be the start of something big.

To begin with, the idea should be spread that you should get a lawyer not only if you are in trouble, but if you wish tostart trouble. The possibilities of doing so are now a well-kept secret in the upper-income brackets, but could be easily dramatized on television spots.

Then, the whole idea of having a lawyer should be associated with being young, glamorous and sexy. Right now, the image of a person with his lawyer walking next to him across a crowded sidewalk is not good.

Picture, howefver, a magazine advertisement in which a beautiful yojng woman is sitting at a plush restaurant, her furs draped behind her, while a handsome man leans toward her. A half smile plays about her lips. "I don't know, darling," reads the caption. "I'll have to ask my lawyer."

Or one which shows a young couple in a meadow admiring her new engagement ring. "Now that we have something to cherish, we should write a will," he is saying.

The same type of couple can be shown later, in their first apartment, which is bare except for a few well-placed antiques. "Suppose we put our rent in escrow?" she asks. "Let's find out our rights, darling."

Or how about a television sequence in which the husband berates the wife for badly done cleaning or cooking chores. A voice-over finishes the scene: Have you thought of legal separation?