George Orwell wrote a famous essay with this title sixty-three years ago. Our political concerns are different from those of 1946 and the piece contains some contemporary references which now strike us as quaint, but, as with nearly everything he wrote, most of his comments on language are as salutary today as they were then, and the clarity and precision of his prose has not dated.
You really need to read the piece, but here are some extracts:
This is a well-known verse from Ecclesiastes:
I returned and saw under the sun, that the race is not to the swift, nor the battle to the strong, neither yet bread to the wise, nor yet riches to men of understanding, nor yet favour to men of skill; but time and chance happeneth to them all.
...and here it is in modern English:
Objective considerations of contemporary phenomena compel the conclusion that success or failure in competitive activities exhibits no tendency to be commensurate with innate capacity, but that a considerable element of the unpredictable must invariably be taken into account.
This is a parody, but not a very gross one. ...Now analyze these two sentences a little more closely. The first contains forty-nine words but only sixty syllables, and all its words are those of everyday life. The second contains thirty-eight words of ninety syllables: eighteen of those words are from Latin roots, and one from Greek. The first sentence contains six vivid images, and only one phrase ("time and chance") that could be called vague. The second contains not a single fresh, arresting phrase, and in spite of its ninety syllables it gives only a shortened version of the meaning contained in the first.
A scrupulous writer, in every sentence that he writes, will ask himself at least four questions, thus:
What am I trying to say?
What words will express it?
What image or idiom will make it clearer?
Is this image fresh enough to have an effect?
And he will probably ask himself two more:
Could I put it more shortly?
Have I said anything that is avoidably ugly?
But you are not obliged to go to all this trouble. You can shirk it by simply throwing your mind open and letting the ready-made phrases come crowding in. They will construct your sentences for you—even think your thoughts for you, to a certain extent—and at need they will perform the important service of partially concealing your meaning even from yourself.
Never use a metaphor, simile, or other figure of speech which you are used to seeing in print.
Never use a long word where a short one will do.
If it is possible to cut a word out, always cut it out.
Never use the passive where you can use the active.
Never use a foreign phrase, a scientific word, or a jargon word if you can think of an everyday English equivalent.
Break any of these rules sooner than say anything outright barbarous.
Nearly all that Orwell wrote here is sound advice for writers who wish to make their meaning clear. Of course, much is written with other objectives in mind: to obfuscate, to gloss, to impress, to amuse, to conceal.... But those who set out to do these things need no advice: such skills, sadly, are easily acquired by almost anyone.