I recall reading a website quite some time ago about the rules and exceptions of multiplication with regards to teaching children. For instance:
The result of multiplying any number times 9 will have a result where the digits add to 9 (1x9=9, 2x9=18 so on, but breaks at 13). It was something like that -- there was a grid and there were about 8 'rules' and 5 'exceptions' that would let you multiply any integer under 13 by any other integer under 13 -- I think.
What's this technique called? Is it a good idea to teach kids multiplication with this technique?