As another poster noted, attend writers' conferences. Google Shaw Guides. You should find listings for conferences there. I attended a conference in Columbus, OH two years ago. In one of the seminars, the editor asked my name, saying that if he ever got a manuscript from me, he wanted to know I'd met him at that conference. Never underestimate the value of contacts.
You can learn so much from other writers. Avoid -ly adverbs in your descriptions. Show...don't tell. Narration has its place but developing characterization, description, and dialogue techniques is invaluable. Check your local Barnes & Noble, which often hosts small fiction writers' groups. Promote your work at every opportunity. Promote, promote, promote.
DON'T misspell. If you want to turn an agent or editor off, send in a manuscript loaded with spelling errors. If an agent or editor tells you to send in a script in a specific manner, send it the way you're told. Prepare for rejection and wear those letters as badges of honor. Some of the most prominent writers in history were rejected time after time.
Someone recommended Bird by Bird to me to read. It was a good resource but one of the best recommendations I got was a book by Reni Browne (I think), How to Self-Edit Yourself into Print. (Or something close to that.)
A friend of mine had written a professional column for a publication and I edited the articles. He made me a much better writer. Editing is a great way--as far as I'm concerned--to develop a feel for how content flows, or SHOULD flow--and develop organization. It's also a great way to follow style guides; if one type of word was italicized, another word of the same type should be italicized.
And advice that I should follow: Dedicate a specific time during your day to sit down and WRITE. Then, do it.