Scott Flansburg, an american dubbed "the human calculator" always tells people that he is "not an athlete, but a mathlete" (Discover Channel-More than Human). When invited to speak on the Oprah Winfrey show, he did a series of warm up exercises before going out on stage much like athletes do before every game and practice. He exercised his mind before the show by adding numbers, multiplying two digit numbers and doing long division in his head. In this Youtube site http://www.youtube.com/watch?v=YGFi-XswpoI, Scott shows his amazing ability to multiply two digit and three digit numbers unbelievably fast. He starts off with a 3 digit number like 373 and adds 373 to itself consecutively and keeps doing it even when he has reached well into the 6-7 digit numbers. His world record is from adding the same number to itself 37 times in 15 seconds (Guinness Book of world Records). It's astounding. He says that he doesn't count from 1 to 10. Instead he counts from 0 to 9. As he astutely mentions, everything relates back to 9. Take 11 for example. The two digits in 11 are 1 and 1. Add them together and they make 2. 11-2 is 9. Try it for 13. It works as well. He says that counting as if  9 was the highest number instead of 10 makes it so much easier. Take the number 55 for example. The two digits in 55 are 5 and 5. 5+5 = 10. 55-10= 45. 4+5 is 9. He says that if you don't get 9 in the end it means you've messed up. Every number problem relates to 9. It's just another way to check if you're right or not.
This is his website: http://scottflansburg.com/