Find a project to work on and someone to mentor you, talk to people at on #haskell and the mailing lists and at meetups.

I learned a lot about writing good Haskell code from working with Yesod. Since it's a framework it structures your code for you and exposes you to a lot of well-designed patterns. Yesod and Conduit are two of the best written packages I've come into contact with (though there are many many more) and simply following their guides has given me new insight into what high-quality Haskell code should look like.

Hard work and practice in your spare time. Probably not what you wanted to hear, but that's the way most people do it.