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 plain 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 thing you are describing. I mean, instead of telling us a thing was “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.
My favorite is #4... it's brilliant advice, but very hard to put into practice. See, I just used that adjective "brilliant," but what I need to do is more writerly work so that you would indeed be dazzled by the brilliance! :-)
Below is an image of a paperback edition the Narnia books from around 1970 - this is the edition I read when I was little. I have very fond memories of those books, and I still re-read them regularly: