Firstly, master the language you are writing in. Know the grammar. Taking a few courses in Latin will almost certainly help. English is a messy language, and learning Latin will provide some wanted clarity. Secondly, try not to repeat the same words too often within a few paragraphs. Spice up your diction - but try not to sound like George Will of the Washington Post either. Use words that most people will know. A reader shouldn't have to sweat to get through your text. Lastly, write only about topics and situations you are familiar with. Don't try and conjure up an Italian mafia family if you don't actually know how they would speak!