Be especially wary of people who ask for your password that claim to be someone doing maintenance, testing or security checking. If you must keep a copy of your password around, encrypt it or lock it up in a safe. To help keep your passwords secret, you should avoid protocols and applications which do not encrypt passwords in stored files or when being transmitted across a network. Use secure versions of these applications such as SSH, kerberos, and SSL at all times. The second most important part of maintaining a good password is to choose a good one.
Short passwords are more susceptible to brute force attacks.
The longer your password the better, I recommend at least 8 characters. A good way to create a strong password is to come up with a phrase that you will be able to remember. Then use the first letter of each word in the phrase intermixed with numbers or symbols. Definitely use more than just letters, use symbols if you can. This way you'll have both a strong password and one that you should be able to remember easily.