That sounds really, really, ridiculously trite. But it's the best advice I can give. It's the best advice I've been given, and it's the best advice I've put into practice.

Read a lot, and expose yourself to diverse writing styles. Write enough to develop your voice, and to recognize it when you find it. Know that there is no such thing as a universally "correct" style, though there are certainly better and worse styles for any task at hand.

Another good bit of advice is to write as though you're talking to someone. Put your conversation down on paper, then go back and pretty it up as needed. First be clear, then be stylish. When in doubt, be clear.