"Review other screenplays."
One of the best teaching tools for a screenwriter is to review the work of other writers from both the aspiring and experienced screenplays. There's no better way to understand standard formatting, tempo, and efficient use of "white space" than to see how other writers accomplish this ... OR ... how they fall short. On those days that you have extra time or need a break from writing: read other screenplays - both spec scripts and those from successful movies!
Two other great websites are Scribe Meets World and Feedback Friday. Both are listed on the Recommended Websites page. Check them out and learn from SMW's script-to-screen analysis and FF's analysis of spec script's first ten pages.