Read, write, and drink a shit load of coffee. Especially the latter.

In all seriousness, practice makes perfect. Read as much as you can (classics are especially helpful), and write everyday, even if it's a little paragraph. Little things like that help keep the mind flowing. Results aren't quick. Like all good things, it takes time. You're not gonna be the perfect writer. No one is. But don't let that stop you from learning. If anything, let that inspire you to improve.

Always remember that quantity does not equal quality. Don't worry if you feel like a post is too short, so long as it gets the point across and it doesn't look like shit. Also, grammar/spelling checkers are your best friends. Lord knows how many times I'd misspell 'necessary' without my friendly, little AutoCorrect.
Most importantly, just have fun. Go with the flow and don't be afraid to experiment. Mess around with similes and metaphors, add a bit fancy descriptions, just have fun! There's no point in forcing yourself to improve if you can't find enjoyment with it.

[hider=Reference Material]
•[url=http://thewritingbeast.tumblr.com/post/104436726621/yo-beast-do-you-have-any-tips-on-how-to-write]Dialogue Tips[/url]
•[url=http://empressium.tumblr.com/post/80963113438/ʜᴏᴡ-ɴᴏᴛ-ᴛᴏ-ᴡʀɪᴛᴇ-ᴘᴜʀᴘʟᴇ-ᴘʀᴏsᴇ]How Not to Write Purple Prose[/url]
•[url=http://chir.ag/projects/tip-of-my-tongue/]Can't remember the word you're looking for? BOOM! Now you can find it with this handy-dandy word searching tool.[/url]
•[url=http://www.hemingwayapp.com/]Hemingway--Editing app for all your editing needs.[/url]
•[url=http://www.springhole.net/writing/marysue.htm]Mary Sue Character Test[/URL]
•[url=http://io9.gizmodo.com/5844988/the-10-types-of-writers-block-and-how-to-overcome-them]Writer's Block is an asshole, so here's some ways to fight it.[/url]
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Remember: Practice, practice, practice. Don't be afraid to break a few rules. Learn as you go. And, most importantly, have fun!