This is a must if you use GMail. If someone gets into your main email account, they can cause a LOT of damage, reset passwords, and all sorts of things. And if you REALLY want to secure your Google account, enable two factor authentication.

Turn on Phone Notifications for Suspicious Activity on Your Google Account
If a hacker gets access to your Gmail account, the results could be disastrous. Google has long notified users via email if their account passwords have been changed or there's a suspicious login attempt, but now you can also set up phone alerts.