In my writing program, we had writing partners each semester, whose work we shared and critiqued each month. It was always a learning experience to find out what we all had been doing all along was very annoying to the reader, or simply didn't make sense.
Here's some unconscious common habits a lot of writers have that put a wall up between their work and their readers -- or worse, a potential editor.
1) Type dot dot dot when your character's thoughts or speech go off into space. Type dashdash when he is interrupted. Type four dots when you want a long pause that follows a complete sentence. Read other common errors in English you don't know you're making.
2) You know how college taught you the high art of the semicolon? Forget all that. Semicolons in fiction are very expensive, don't use too many on a page. Also, remember that. Short sentencs. Equal. Action!
3) You might not be an outliner, but I don't know any novel writer who doesn't have to do some written down prewriting. Read books and websites about structure and plotting until you find the outlining/worldbuiling method that works for you, because nothing is worse than your plot going nowhere, or your villain's name or species changing every 50 pages. Some people use writer's software, some do an encyclopedia of their world in Word, some use Q&A format in Excel, some hang up poster boards using actors and magazine landscapes. You need something other than your mss to keep you on task.
4) Join a professional guild and consider attending conferences. It doesn't have to be exactly what you're into -- ie, your average romance writer's group will welcome fantasy and women's fiction writers. It's invaluable to know what's new in you genre and, perhaps more importantly, what's so old it's unsellable.
5) "Said" is fine as a dialogue tag. "Blah blah," Beatrice said is fine to put down 50 times in a conversation. "Said" is one of those words that readers skip over, almost like a punctuation mark. Similarly, do not use 10 synonyms for "said" because repeating "said" feels repetitive to you. Of course it's repetitive - that's why readers skip over it and pay attention to the dialogue. Do you want your prose to be awkward and obvious?
6) More on dialogue: Character voice is the most important element of your book to lending personality to your characters, but unfortunately our TV culture has corrupted how we "see" conversation in the written format. Find other ways to convey your character's mood than using adjectives to describe their voice, because that way leads to talking heads; you are not writing a script, there will be no actors putting life into your scene, pages of dialogue and "-ly" lead to boring talking heads.
7) Consider every single bit of concrit you ever get. Humble yourself. Find other author friends and solicit their opinions.Take classes at adult education centers or libraries. Read books on writing and do the exercises. Write something every day.
7) Keep your first rejection letter. Frame it. Throw it a little party. It means you're a real writer.
Photo credit: Old typewriter by Petr Kratochvil