There are actually a few good articles on that topic at PCWorld:
http://www.pcworld.com/businesscenter/article/170662/the_art_of_creating_strong_passwords.html
http://www.pcworld.com/article/154521/why_you_need_a_password_manager.html
and many more.

I personally use KeePass with a strong but memorable password to manage most of my "online" passwords as you call them.
For "offline" passwords I use a simpler Passphrase in combination with a YubiKey (http://www.yubico.com/yubikey), e.g. to encrypt my laptop's HDD or my encrypted file-containers. The YubiKey provides (among other options) a 16-64 character static password. Of course, having to rely on a physical device like a YubiKey brings also some riscs. If you lose the key you're pretty much screwed, that's why I recommend either ordering a second YubiKey and program it with the same static password or writing the password down and storing it in a safe or a lockbox at a bank of your trust. But the most important thing is to never use the YubiKey's static password alone, you should always use it in combination with a leading passphrase so if anybody should actually steal your YubiKey they don't get access to anything you used it for.

To test the strength of your passwords you should always check them at http://www.passwordmeter.com/.