What finally clicked for me was consistently doing the weekly review and making use of the "Active" status and "On Hold" status.
It's nice to be able to sort everything into place the first time around. But if you let the weeds grow, it'll become unmanageable.
Every week, go to review mode to refresh yourself with all your projects and tasks. Either delete a project/task, delegate it to someone else who may be better suited for it (they either have the time and/or the expertise to complete the project/task) or keep it on your list.
By doing the weekly review, you'll feel confident that everything is still relevant - all your projects/tasks are still meaningful.
If a project/task has lost its value (because the time has passed or priorities have changed), then you may as well just delete it.
I weed out all the irrelevant projects every week (it was a nice idea at the time but three weeks later, I realize that I really didn't want to do it anyways).
You may also try using a Someday/Maybe folder and put projects placed in that folder with that "on hold" status.
I usually put all new projects that I brainstorm up in "on hold" status and place it into the Someday/Maybe folder. I already have enough "active" tasks to keep me busy.
In the weekly review, I'll review my Someday/Maybe projects and see what I can delete. Sometimes a project interests me again and I'll take it out of Someday/Maybe and set the status to "Active."
Check out
http://usingomnifocus.com/ and get the book. It's well worth the money spent on that book. You'll probably find a lot of forum posts praising the book.....