Use strong words. Good writing is precise, evocative and spiced with the unexpected. Finding the right verb or adjective can turn an uninspired sentence into one people will remember and quote for years to come. Look for words that are as specific as possible. Try not to repeat the same word over and over unless you are trying to build a rhythm with it.
One exception to this is the words used to describe dialogue. Bad writing is filled with "he commented" and "she responded." A well-placed "sputtered" can work wonders, but most of the time a simple "said" will do. It may feel awkward to use the word "said" over and over, but changing it up unnecessarily makes it harder for your readers to get into the back-and-forth flow of the conversation. When writing dialogue, you want readers to hear your characters' voices, not your own.
Strong doesn't mean obscure, or more complicated. Don't say "utilize" when you could say "use." "He sprinted" is not necessarily better than "he ran." If you have a really good opportunity to use "ameliorate," go for it—unless "ease" is just as good there.