Protect your domains from theft

Ken already provided some fantastic advice on domain locking, I'd suggest the following computer security items to avoid the bad actors from taking over your email, unlocking your domains, and perpetuating other mayhem.

Secure your computer. Much of the malware out there is trying to take over your computer to extract information about your online activities. While stealing domains isn't at the top of the list, they've long looked for FTP credentials to add malware to existing websites.

• Consider using OSX or Linux. While these aren't unhackable, they don't have an abundance of automated exploits attacking them.
• If you'd prefer using Windows, the 64 bit variants of the latest version of Windows are generally more secure.
• In addition to operating system updates, keep your software up to date. Most attacks these days are against Flash, Acrobat, Java and other browser plugins, not against the operating system or browser itself. Secunia PSI is a useful tool for keeping your Windows software up to date.

Use secure password policies.

• Don't reuse passwords on multiple sites. Use tools like Keepass, LastPass and the like to manage passwords.
• use long passwords with lots of variety in characters or use pass phrases (a sentence)
• Take a good look at the security questions some sites use for password resets. Is your mother's maiden name and your father's middle name exposed on some genealogy site somewhere? Does your Facebook timeline expose your first girlfriend's name or your favorite pet's name? A good password manager will let you store passwords for these kinds of questions, consider randomly generated passwords for these security questions instead of answering the specific question.

Use additional security tools available to you. Google offers 2 factor authentication for Gmail, if you rely on Gmail enable it. Make sure SSL and similar technologies are enabled on all your email accounts, especially if you connect to public networks (coffee shop, library, airport, etc).