Buy DNS backup service. A lot of downtime (and headaches) can be attributed to problems related to your DNS. DNS backup services constantly grab your DNS data and act as a backup if your primary DNS goes down.
Buy a monitoring service. You can purchase a service that pings your website every few minutes and notifies you (via text message, email, etc.) if it goes down.
Always backup your database. In addition to making regular backups of your website and databases, make sure you create an additional backup before tweaking the database itself.
Make sure your domain name registration is up to date. So many downtime fiascos could be solved by simply remembering to renew your domain name. Go ahead and set your domain name to auto renew. Or purchase your name for the next ten years and set the domain registrar lock.
Use Google Webmaster Tools (GWT). Using GWT is a no-brainer. It provides you with detailed reports about your pages’ visibility on Google and will notify you of any errors that are encountered while crawling it.