Practice writing that has an observable effect. For example, write Facebook statuses and see who "likes" or "comments" on them. Think about how you write emails and to whom. See what styles and contents tend to be effective. Try different variations.

(Don't draw conclusions too soon; there is a lot of random variation in responses.)

Also I agree with what everyone else recommends. But, in my experience, there's no substitute for these real-world experiments that appear in my day-to-day life.