Is it really true that you should replace your spices every six months? We've never done this, because it would cost a fortune. Also, we've found that although some spices lose their zing after a few years, others hold up just fine for a very long time. We moved a container of crushed red pepper flakes from San Francisco to Sonoma County to Oakland to Connecticut, and those flakes packed a punch right up to the end.
Salon addressed this issue a few days ago, and it's worth reading. The author not only delves into the six-month question but gives excellent tips for getting the most out of your spice investment. In a nutshell: buy them whole, and grind them as you need them. Ground spices give up the flavor ghost much sooner than their whole counterparts, because their essential oils dry up.
"Buy whole" is good advice, and we try to do that. Overall, though, we hang onto our spices as long as we can and replace them as we need to.