As far as replacing helmets is concerned, it is my understanding that the foam degrades and at 3-4 years you're looking at a pretty structurally compromised helmet. It'll protect you from small hits but a big hit with an older helmet...not so much. So I'd probably say every few years pick up a new one. If you're got any reason to question whether or not you should (from a hit), check the foam and if you've hit hard enough to crack the foam or the shell even a little bit, dump that ish. If not, you're probably okay for the time being, but definitely consider. You can never really check the whole helmet for cracks anyway.
If you hit your head (with your helmet on) hard enough where you thought "I'd have been so fk'd if I didn't have my helmet" give it a good once-over to make sure it isn't boned.
You can see if Giro will provide you some extra pads - for my bike helmets, they've always come with some extra padding that you can stick inside (adhesive) and help shim the helmet a little bit if your head isn't shaped perfectly for it.
Last edited by vltsai; 01-07-2013 at 02:32 PM.