Actually, I will give TWO!
First, speak slower! Most foreigners speak much too fast. That is why their English does not sound clear. If you speak too fast, you cannot say all the sounds in a word. (Also, speaking slower gives you time to think!) So – SLOW DOWN!
The second tip is this: THE BEST ENGLISH IS THE MOST SIMPLE ENGLISH! Simple, short sentences sound very elegant and make your life much easier. You must know how to understand long, complicated sentences, but when you speak, you can never go wrong with making it simple. If you look at the writing of the great American author Ernest Hemingway, you will see that his sentences are short and simple. You can’t get better than Hemingway!
Every language has its own “key” to elegance. In French, it is the choice of this particular word as opposed to that particular word. In German, it is the logic of the grammar. In Chinese, the right four-character phrase is a thing of beauty. Good Japanese has long sentences that build up many ideas and then “tie them up” with the verb at the end. In Spanish, the right use of the subjunctive makes a simple sentence poetic.
In English – making it clear and simple (and slow!) is always a good idea.