What am I talking about? Practice. Oh! The anti-climax. However, knowing to practice and actually doing it are two different things. Here are my suggestions for making practice enjoyable.
Start a blog – it’s easy. [At wordpress.com you don't even worry about hosting. Pop in your name, and you're off.] Blogging gives you creative freedom. You can write about any topic; from astronomy, to your sad life as a man who makes yogurt-pot-peely-tops. And, you can write whenever, and from wherever, you like.
Get involved in Wikipedia – Wikipedia is written by volunteers from around the world. Registering takes, literally, a minute. And then you are free to edit. You can join a interest group. Or, join the official grammar patrol. Or, go it alone.
Writing for Wikipedia is: fun, gives something back, and there are no commitments.
Join a writing forum – an online forum (message board) is an internet site where you post messages and other users can reply. Writers’s Dock is one example. You can join competitions. Have your work reviewed. Review work. Or, just socialize.