I'm the master at cracking helmets...at least when I'm working on them that is :confused . Asok is right, it all depends on what material it's made out of.
Then I'd recommend a nice thick layer of 2-part epoxy Plastic Welder. That stuff sticks like crazy to anything, is a tad flexible, and can be sanded and painted. I get mine from walmart and it's made by Devcon or Devron??...something like that.