If you insist on doing so, best practice would be to ensure you’re using a firewall, up-to-date antivirus and anti-malware, etc. Become extra diligent about opening attachments, clicking links, visiting web sites with which you aren’t familiar, all the other ways that attackers deliver their exploits to you. Minimize exposure to the Internet. If you normally leave your computer on and connected all the time, start turning it off when you aren’t using it. You might even go as far as to disconnect from the network when you’re using it for non-Internet activities, such as composing a document in a word processing program.

It’s likely that when most people do upgrade and XP becomes a tiny percentage of operating systems in use, it will become a less attractive target and maybe the bad guys will lose interest. So even if you refuse to upgrade, you should hope that everyone else does.