Hi Gully,
Good question! For some time manufacturers have suggested you need to replace every few years, but I've not seen evidence that a helmet stops protecting you if it's undamaged (marketing hype?). MET suggest helmets should last at least 8 years.
There are obvious times when you must replace - after a crash, for example, but that should be obvious.
Check out this useful link:
http://www.helmets.org/replace.htmAs for suggestions, if for MTB something with greater side/rear coverage is useful just in case! I saw one crash where greater side coverage certainly saved more serious injury (rider still had concussion and a cut ear though). Other than that, it's down to styling, fit and adjustments, colour and any bling factors you choose to throw in!
Steve