There’s a very helpful piece on the USA Today site about ways to preserve your frequent-flier miles as the airlines shorten the expiration dates on inactive accounts.
I have credit cards affiliated with the two airlines where I keep active miles, and I usually fly on one of them or a partner airline that will credit miles over. This article, by Gary Stoller, offers some more creative strategies.
If you might be in danger of losing your miles, check your account. Sign up for e-mail updates, so you can track it. And read the article.