A good idea is using any of the scripts linked below to generate passwords for sites. Basically, you need to choose a master password and supply a site's domain name. The script concatenates the two text strings and generates a MD5 checksum on it and you use the result of that as password for the site. This way you use different passwords for all your sites but don't have to remember more than one password.
There are different versions of this simple idea and I've linked two below. There are greasemonkey (firefox plugin) and bookmarklet versions as well that you might want to check out.
(The different versions are usually not compatitable with each other. Be sure you know which one you're using.)
http://www.angel.net/~nic/passwd.html (the original?)
http://www.hashapass.com/
/curtanwoo
EDIT: bah spelling
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