You can also get more from OmniFocus by also reviewing and re-reading David Allen's books, Getting Things Done and Making It All Work.
Every once in a while, I like to open the books and review it. I'll often catch something that I didn't get before. It's like reading a Shakespearean play. You'll find something new when you're reading it again after a few months of digestion.
At this moment, I'm reviewing Making It All Work and slowly working my way up the different levels of elevation. I knew I was comfortable with the runway level (next actions) and the 10,000 feet level (projects). A couple months ago, I just started to get comfortable at 20,000 feet (Areas of Focus & Responsibility). Maybe in a couple more months into the future, I'll get comfortable with the next level at 30,000 feet (Goals and Objectives).
As I get more comfortable with the next level of elevation, I start tweaking my OmniFocus workflow to fit my newfound revelations.
OmniFocus will be futile unless you learn to master the fundamentals.
Another book that actually catapulted me higher is the Zen-To-Done handbook found at zenhabits.net
http://zenhabits.net/2007/04/zen-to-...tivity-system/
http://zenhabits.net/2007/11/zen-to-...tivity-e-book/
This alternative approach to GTD actually helped me tenfold and improved my grasp on GTD. I worked my way through the 10 habits outlined in ZTD and feel great about mastering the concepts.
Every time I re-read GTD, Making It All Work, and ZTD, I actually start seeing things differently. Then I tweak my OmniFocus setup.
So, while getting an OmniFocus manual is great, sometimes it makes sense to return back to the basics. :-D