Del Warren and Carolyn Dorin-Ballard discuss the importance of the pre-shot process. The key is understanding that what you do behind the approach is a completely different function than when you actually get up and throw a shot. They are two separate processes: thinking box and playing box.
The mistake many players make is not thinking through their ideas from the thinking box completely by the time they step on the approach. In this video, watch them demonstrate the difference between the two processes.