Many years ago, I went to my first ever writing seminar. It was called "Writing the Breakout Novel", and it was run by a literary agent named Donald Maass. My experience there represented a major turning point in my own writing - because, even though I wasn't ready for the big time then, I learned some things that allowed me to improve my craft by leaps and bounds.
Now I'm represented by Cameron McClure with the Donald Maass agency, and I have to give some of the credit for getting me here to that seminar. Don's writing advice is fantastic. And the good news is, he's giving some of it away for free.
You can download a copy of The Career Novelist by Donald Maass here.
I also highly recommend that every writer check out Don's books "Writing the Breakout Novel" and the companion "Writing the Breakout Novel Workbook." You'll learn so much about writing, your head might just explode. :-)