Do a search on the various forums and you will find many posts dedicated to this topic. E.G., just yesterday this question was asked in this forum at http://www.flyertalk.com/forum/miles...xpiration.html (page 2)

I would search the forums you are concerned about particularly. Generally (but not for all), some rewards activity is enough to renew an account (e.g. a credit card purchase with your reward account) -- but you need to know the specifics of your programs. SPG is different than AA for example.