I've heard that temperature stability is really important, besides a dry environment (which is easy - put in ziplock bag) also lower temperature is important too. Different seeds are good for different amounts of time - some 1 year - some 5 years no problem. As they get older you'll have less success with them, but there's always a chance some seeds might be good even when really old.
I've started letting some things in my garden go to seed, and dried and gathering them just to learn how. Also planted 3 cilantro seeds - and all three germinated! Pretty fun stuff, and I think useful too.