1) Read a lot. Read different styles. Read the newspaper, read fiction, read blog posts, read history textbooks. Read lots of things. The diversity of styles will give you ideas on how different things can be communicated well in their own context.
2) Analyze what you read, just a little. I started with just asking myself "who is the intended audience of this writing?" Asking a consistent question in your analysis will make comparison easier. It will also help you think through your own writing: "am I writing this for the marketing person who is going to try to sell it? am I writing it for the coder who has to install it? maybe I need two separate documents for each of them?" That thinking about writing will help you analyze and adapt your own work.