For example, http://facebook.com/howchoo (shameless plug)
Under the main blue navigation bar at the top of the page (the one containing the Facebook logo and search box), you'll see a lighter blue box that states something to the effect of:
You are posting, commenting, and liking as howchoo ? Change to Zach Levine
Click "Change to [Your Name]" and you can now post, comment, and like your page's content using your personal Facebook account.