This piece from Anne Holmes at the New York Times points out mistakes she looks back on and wishes she could have avoided all together. In no particular order, they are:
1) First person in reported narratives
2) Long, complex sentences that make you seem intelligent.
3) Assuming one should use aggressive, masculine prose only because that's what people like.
4) Taking too long to get to any sort of point
I found these "errors" really interesting--I often feel like I'm still an amateur author, and that I, too, make these mistakes. Particularly the 2nd one. I often write incredibly long sentences, and make them even longer, but when I return, I realize that there is no reason they are long. Short sentences are good, too.
Useful tips to remember as we try to write more!