I think it's important for you and I to become good at GMing without scripted or published materials because you never know what your players are going to do. Almost every session I need to depart from my planned notes and make things up quickly for a new encounter, NPC, world detail, etc. Becoming good at that will definitely improve the quality of our sessions. A key to winging-it successfully is maintaining game consistency. It's important to keep track of what you've invented and to play it correctly in future game sessions.
Players are adept at picking up mistakes and inconsistencies.
And your campaign as a whole will suffer. By left-brained, I mean I like taking notes, making lists, using charts, being organized and analytical. I suspect you will fall somewhere between left and right-brained, so perhaps you should read the tips for both groups and pick the ones that suit you. The article for Fiction-Fantasy.net should be out this week, and I'll confirm it next issue. On Tracking Key Information Learn to track only the information that is critical to campaign consistency.