Make two logons: admin and user (Score 1)

This is a big one for me, and I think it's made my machines more stable. On first setup, I make an admin account. You can name it anything (admin, superman, BillyBob, x, etc.). Use that account only for installs and upgrades. Then make another account you'll use every day. Don't give the everyday account admin privileges. That means any slimy code you get while surfing won't have admin privileges, either. (More knowledgable geeks correct me on this if there's a chink I don't know about.) Account switching feature in OS X makes it easy to pop over to the admin acct if you need to get the latest release of something installed. Also, if it's not a simple app, and always for OS X updates, I first logout, reboot, run Repair Disk Permissions (Applications > Utilities > Disk Utility), do the update, run Repair Disk Permissions again, and reboot. That makes it pretty certain that major updates are happening in a clean environment.