Since I am an author myself, and a junior literary agent, there are things I've noticed about authors and how they roll.
I've noticed that one of the number one things authors do to hurt themselves is being too impatient. We do not want to wait the time we need to make sure our manuscript is perfect. I've seen it way too much in my own work and others. The thing that works best for me is writing, putting it down then picking it back up in a few weeks or months. You can go back and will see mistakes that you didn't right after writing.
Also, we also need to have someone look at it that isnt our family. As much as we love for Mom and Dad, Brother and Sister to enjoy our work, we need someone that reads a lot or does not have a relationship with us. In other words, someone who will not tell us our work is great when it actually sucks. I've heard and done this too. It's easy to do, but so much better if we don't and get an honest opinion.
Something I see way too much, is author's not wanting to get help from others, like agents or editors. Everyone likes to think that their work is great, but of course it makes sense to you! YOU WROTE IT!. My Intro to Nonfiction Creative Writing teacher broke me of that habit quickly. We need others opionions and we need to realize that not everything we write makes perfect sense to others. It's never perfect. Believe me.
These are just some things I've picked up on as I observe. We all need to stop and observe the things we do. Get others opionions and give our work some time. Stop being so impatient. Once our work is ready it will have it's time.