When I fully figure out what would be the best advice to give I will give it. This will be a work in progress as I continue to work on my craft and expand my knowledge.
First, you must write as much as possible. Readers read, thinkers think, and writers write. Even if you don't have that much time, just get a little writing done. I write every day and have since September 2009. Sometimes I get a few pages of a script, or a page of a novel written, when time is scarce I might only get a few lines down. But I'm still working and that's important to me.
You should read a little of what you'd like to write. Experts say you should do this all the time, but my time is limited so I read when I can. If you want to write children's books, read them. If you want to write novels, read them. If you want to write screenplays, read them. You get the idea. You will get a feel for the structure and how a typical piece of writing looks.
When I start a new project, I will usually read a little to get me in that mindset. I'll also look at that book or whatever and my writing to make sure it reads "right."
Most of what I've written are screenplays, so I will focus mainly on that. When I start a new script, I will usually find a movie in that genre, watch it, and write down the scenes and the times they happen. It makes me focus on the structure, plot, and pacing of the film more and makes me more aware of it in my own script.
The screenplay needs to look like a screenplay. I recommend the excellent book, "The Screenwriter's Bible," by David Trottier. It's a tremendous resource for the structure of a screenplay and I still pull it out when I have questions. This is also where reading screenplays helps, so you can see what it's supposed to look like, and if it doesn't look right, it doesn't get read.
