Read.

No, seriously. Read like crazy, especially writers whose work you enjoy. Try and figure out what you enjoy about that person's work, see the tricks they use and the styling that they employ. Plagiarize these things furiously. (The techniques, not the content unless you like pissing people off...) At the same time though, try and add your own personal 'thing' into the mix to give yourself a distinctive style. Don't worry too much if this doesn't come straight away, it's something that will naturally come with practice - the more you write.

Set yourself challenges as well. It's all well and good if you can write a five-hundred page essay on why, say Half Life 2 is the best game in the world. But can you do that in five hundred words? Can you do it in fifty? Bigger isn't always better and don't be under any impression that bigger words means someone's a better writer - the best of them can say the same thing more succinctly.

Most of all, be receptive to criticism. I don't mean the rubbish from internet commentators with their "OMG THIS SUX LOL" - if you're writing for a good outlet/publication, the editors will point out faults and problems if there's something wrong with what you're writing. Don't be afraid to ask for advice if you need it from them or other writers. Also don't be afraid to look back on your older stuff and determine what you liked and don't like about it. Then do more of the former and less of the latter.

At the end of the day, you can only improve by doing. So don't be afraid to keep trying and don't suddenly think you've done enough once you are getting paid. Would I say I'm a great writer? No way, not in any sense - to the point where I'm not sure I'm really qualified to be answering this question. But I appreciate being asked immensely.