I'm a fan of carrying a half dozen little zip ties. Very versatile little things!
As far as rear derailleur failures... for the same reason that you should never say "I can't remember the last flat tire I've had", you should also never tempt the gods of fate by boasting how you haven't had a derailleur break in a long time.

Steve in Peoria
(my last rear derailleur spring failure was just last year, and it required a 3 mile walk in cleated shoes, with the bike slung over my shoulder. )