Back up your data... twice
Backing up your data may not seem like a security matter, but guarding your data against faulty hardware or a devastating virus that wipes out your hard drive is extremely important. For best results, don’t just back up to an external hard drive at home. You should also back up to an offsite location to protect against fire, burglary, or children carrying water.
Online backup is the easiest way to get offsite protection. The numerous services available include Backblaze, Carbonite, CrashPlan, iDrive, and Mozy. A disaster-proof backup scheme doesn't have to cost you a dime, though, if you're willing to do a wee bit more legwork.