1. Even if you brief in person it’s always a good idea to confirm the main points in writing. That way you both have something to refer back to.
2. Apart from tackling creative content, your brief should include a simple description of the approval process, time frames and confirmation of prices agreed.
3. Try showing your written brief to a colleague and asking them to describe back to you what you want. If they can’t tell, a stranger certainly won’t be able to.
4. If you want something specific, say you do and describe it as best you can. Show them examples of the type of things you like. But then ask your designer / copywriter for their honest opinion of what you’ve asked for.
5. Still on specifics, don’t forget to cover the basics: size, whether you want landscape or portrait, the inclusion of any logos or corporate font sets etc.
6. Keep the length of your brief in proportion to the size of the job. A £100,000 campaign deserves a ten-page brief. A £100 flyer doesn’t.
7. If you’re not sure what you want, it’s acceptable (indeed preferable) to ask the creative to produce a rough draft – a sketch or a few sample paragraphs of copy – before progressing…
8. …But pay them for it. Good copy and design may look easy. It isn’t. It takes time and deserves reward.
9. Experienced creative people are practiced at interpreting briefs, but they can’t read minds. Encourage questions and think carefully about your responses, because…
10. By definition creative people think laterally, so inevitably they’ll tackle any brief the same way. The best briefs pin down the necessities while encouraging creative freedom.

Here’s a little bonus tip:

Sometimes, it’s just as useful to know how not to do something. This article, entitled “How NOT to brief a graphic designer” has been circulating designers’ in-boxes. Enjoy, or cringe.