David Ogilvy is one of the most famous 20th century advertisers. Here are some of his rules for getting and keeping clients.

Avoid working with clients who have a different ethos to you. It’s far easier to advertise a product or company you genuinely believe in than one you don’t. Similarly, avoid clients who erode your staff morale.

Never work for a client so big you can’t afford to lose them. Why? The fear of losing them will wreck your life. You will be unable to give candid advice. Ogilvy famously turned down Ford, saying: “Your account would represent one-half of our total billing. This would make it difficult for us to sustain our independence in council.”

Never send a letter or a memo on the day you write it. Read it aloud the next morning—and then edit it. Before you send your letter or your memo, make sure it is crystal clear what you want the recipient to do.

Use short words, short sentences and short paragraphs. Never use jargon words like reconceptualize, demassification, attitudinally, judgmentally. They are hallmarks of a pretentious ass.

Remain flexible when selling clients. Rehearse before meetings, but never speak from a prepared text. A slide deck or written text locks you into a position which may become irrelevant during the meeting.

Above all, listen to your clients.

Learn how to make presentations. People who think well, present well. Woolly minded people write woolly memos, woolly letters and woolly speeches. Good presenting is not a natural gift. You have to learn to present well.

Don’t take on clients whose business is dying. You may be hungry for work, but the success of your clients defines you. Your profit margin is too slim to survive a prospective client’s bankruptcy.

It is important to admit your mistakes and to do so before you are charged with them. If you draw client’s attention to your flaws before the client notices them on their own, you will also gain client’s trust. Brains are not enough unless they are combined with intellectual honesty.