In reply to Sempafi:
I'm surprised no-one's mentioned Bicarbonate of Soda yet - I've always sprinkled a thin layer on the inside of my shoes, given them a shake, and left them for a few days, once every few months, and it's always worked surprisingly well.
The downside is getting it all back out so as to avoid having bits digging into your feet when you use the shoes again - I always use a vacuum cleaner with a thin nozzle.