What's the difference between God and Pilots? God doesn't think he's a Pilot. (Sorry, that was kinda lame).
In response to the original question, there's a little more info here on the types of things that are tested/checked: http://www.dciem.dnd.ca/medical/glfs_e.html
There's some stuff you can kinda control (weight, colesterol, blood sugar, heart condition, etc.) and other stuff you really can't (vision, height): be fit, eat right and pray for the rest ...