What are some ways to maintain intelligence with age?

There is a general consensus that you can't increase IQ, but it seems that we can still do things to maintain it with age like exercise. I even heard about a study claiming that learning a second language delays the onset of dementia, but I'm not sure if the result stands up to further scrutiny. What are some other things that I can do to stave off cognitive decline? The more varied the methods, the better.