To do this, you only need to have 25 fans (it used to be 100).
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Go to http://www.facebook.com/username but be careful with this next bit- you want to set a username for your page and not for your personal facebook profile, and this can't be changed once you've set it. There should be an option towards the bottom of the info that allows you to choose to set a username for your page.
Now you'll have an address for your fan page that looks more professional and is easier to share!