Pretty much what's already been said - read and write. Don't just limit yourself to your personal favourites, though. Read widely - different styles and genres, fiction and nonfiction, and so on. Hell... even reading bad writing and giving some thought to what makes it bad can have value.
For writing - try to write regularly. If you can't actually think of anything to write, then there are various exercises you can pick up from writing courses, or books on writing, to help you get things moving. One that tends to work for me is a 'stream of consciousness' writing exercise that I got from a creative writing course I did at university.
Set a time limit and just start writing and don't let yourself stop or pause until you're done. You'll probably spend a fair amount of time writing about how stupid you feel, or how you can't think of anything to write, but there's also likely to be stuff mixed in there that you can pull out and use.