Arthur Benjamin is both a professor of mathematics and a magician. After wowing the audience with some amazing feats of calculation, he is happy to reveal his secrets. Seemingly impossible problems can often be broken down into more tractable steps and phonetic codes that can be held in memory and processed quickly. Benjamin explores these steps, giving examples of how he is able to multiply large numbers together, or pinpoint the day of the week of any birthday in any year.

You have to listen to this act of his on IT Conversations.
http://itc.conversationsnetwork.org/shows/detail3501.html