If you like this kind of stuff, there are a couple of fun mental math books I can recommend:

http://www.amazon.com/Speed-Mathematics-Secret-Skills-Calcul...

http://www.amazon.com/Secrets-Mental-Math-Mathemagicians-Cal...

It's really baffling that people don't teach this stuff in school.

I try to spend a half hour or so a day playing around with Bill Handley's Speed Mathematics book (http://amzn.to/cE5hDu)

The techniques aren't quite as homespun as chunking/gridding, but they're definitely fun.