api design how to handle stale data in cached responses

So I've been chewing on this API caching thing, right? You know, the kind where the data just won't sit still. We've got Redis for our user stuff, and yeah, it's zippy, but making sure it actually matches what's *really* going on? That's a whole different ballgame. How are you guys handling old data? Are there any API design tricks that help, or is it all about kicking the cache out when it gets stale?