What makes a strong password

To an attacker, a strong password should appear to be a random string of characters. The following criteria can help your passwords do so:
Make it lengthy Each character that you add to your password increases the protection that it provides many times over. Your passwords should be 8 or more characters in length; 14 characters or longer is ideal.
Many systems also support use of the space bar in passwords, so you can create a phrase made of many words (a "pass phrase"). A pass phrase is often easier to remember than a simple password, as well as longer and harder to guess.
Combine letters, numbers, and symbols. The greater variety of characters that you have in your password, the harder it is to guess. Other important specifics include:
The fewer types of characters in your password, the longer it must be. A 15-character password composed only of random letters and numbers is about 33,000 times stronger than an 8-character password composed of characters from the entire keyboard. If you cannot create a password that contains symbols, you need to make it considerably longer to get the same degree of protection. An ideal password combines both length and different types of symbols.
Use the entire keyboard, not just the most common characters. Symbols typed by holding down the "Shift" key and typing a number are very common in passwords. Your password will be much stronger if you choose from all the symbols on the keyboard, including punctuation marks not on the upper row of the keyboard, and any symbols unique to your language.
Use words and phrases that are easy for you to remember, but difficult for others to guess. The easiest way to remember your passwords and pass phrases is to write them down. Contrary to popular belief, there is nothing wrong with writing passwords down, but they need to be adequately protected in order to remain secure and effective.
In general, passwords written on a piece of paper are more difficult to compromise across the Internet than a password manager, Web site, or other software-based storage tool, such as password managers.
http://www.microsoft.com/athome/security/privacy/password.mspx