C. S. Lewis once wrote to an American boy with advice on clear writing. It turns out to be excellent advice for pastors on preaching as well:

1.   Always try to use the language so as to make quite clear what you mean and make sure your sentence couldn’t mean anything else.

2.   Always prefer the clean direct word to the long, vague one. Don’t implement promises, but keep them.

3.   Never use abstract nouns when concrete ones will do. If you mean “More people died” don’t say “Mortality rose.”

4.   In writing, don’t use adjectives which merely tell us how you want us to feel about the things you are describing. I mean, instead of telling us the thing is “terrible,” describe it so that we’ll be terrified. Don’t say it was “delightful”; make us say “delightful” when we’ve read the description. You see, all those words (horrifying, wonderful, hideous, exquisite) are only like saying to your readers, “Please, will you do my job for me.”

5.   Don’t use words too big for the subject. Don’t say “infinitely” when you mean “very”; otherwise you’ll have no word left when you want to talk about something really infinite.

(From: Letters to Children, Dorsett & Mead, editors, Simon & Schuster, New York, 1985)