Unless you're trying to get into the account of somebody semi-computer-literate, try their username as the password. If that doesn't work, try "password" or their real name. (Usually just their first name and usually all lower-case.)

If that doesn't work, move on to a script that will check all dictionary words, then a brute-force attack.

Many people aren't good at picking passwords and you could very well get in. So don't even try guessing. Privacy is a Good Thing. Protect it.

My favourite excerpt from a password cracking FAQ:

What about login password in Windows 95?
Have you tried to press "Cancel"?