Welcome to FT!

NW miles never expire, although the airline has the option to zap them after three years of no activity.

I think CO miles expire after 18 months of no activity.

I am not aware of HH points expiring.

The best way to keep an account active is idine. You sign up online or on the phone. Basically, you register up to three credit cards per FF account, and then once per month at participating restaurants you get up to ten miles per dollar spent on registered cards. It's a great deal.