Another great screenwriting resource is screenwriting professor Scott Myers' website Go into the Story. I just discoverd it this week and found a lot of helpful tips.
He's running a great series on agency coverage, including the politics of said coverage. This is an area I've generally stayed away from since I don't want breach any confidentiality agreements I've signed over the years. As you'll see, while agency coverage is very similar to standard coverage, there are a few other factors that an agency reader might have to keep in mind as part of their job.
Anyway, check out the site. I'm currently going through the archives and enjoying many of the posts.