I was talking to my dad the other day, and he was saying how he'd watched a documentary on this a while back (don't know the name, sorry). One of the more interesting methods of doing this is actually to learn how to properly use an abacus to work out the multiplications, divisions and additions.
The documentary was all about a class of kids that trained their mind on using the abacus for a couple of hours a day - I think it was in one of the Eastern countries. After a couple of years of doing this, they were able to abandon the abacus completely and just picture it in their minds. These kids are able to multiply two 5 figure digits in their head and give you the answer within a couple of seconds.
Now, I'm not saying that you should do this - it's probably never going to be necessary for you to multiply large numbers in your head - but it does give an interesting insight into how powerful the human brain is if applied correctly.