For your semi-important domains, you should sign up for the FREE Snap Names monitoring service. They email you if there are any changes to your domain registration, including expiring, change of owner, etc.
For your really really important domains, you should spend the $69 and sign up for the Snap service that will monitor them and attempt to grab them if they drop. It's worth it for a few bucks.
http://www.snapnames.com/