you can also use a hairdryer. The chemical reaction that takes place in the resin never really quits, so it'll stink for weeks or months. If you heat up the helmet with a hairdryer, it'll reactivate the resin to hardener reaction, and drive off alot of fumes. The babypowder also works pretty well, but you might have to do a reapplication after a few weeks, depending on weather and such. If its hot outside and you're wearing the helmet, you'll start smelling the fumes again.
However, don't get it too hot if you're using a hairdryer. It'll start to bubble, and you run the risk of it catching fire. Trust me on that one.