Pitching certainly makes up a large part of that 90 percent half mental. You can have great mechanics and natural ability, but none of it will matter if you have a "ten cent head." Learning how to think on the mound is a significant part of a pitcher's training. I have a couple of standard lessons devoted to the subject myself, covering such topics as thinking simple, focusing on the task at hand, dealing with negative thoughts, blocking out emotions, developing routines, etc.
Here is a recent article from the Youth Sports Psychology Blog about helping young athletes focus and block out distractions when competing. It also discusses the advantages of having a pre-game routine. Routines, before or during competition, lead to comfort and confidence (no matter how ridiculous they may seem).
Blocking out distractions is a necessity. NLCS Game 3 starter, Chris Carpenter, will work around the distractions tonight, despite the Brewers' best efforts. According to Carpenter, "It's about eliminating the distractions. If you can't eliminate those on your day, you're going to have a difficult time. I'll go out there an have my game plan and execute the best I can." Last time out Carpenter found the "zone" he needed to shut out the Phillies and send his team to the next round of the playoffs.