view site as different user?

I'm trying to work out the best way of allowing a staff user on our client's website to be able to quickly view the site as a different user type. Different content is displayed depending on which user type is logged in. At the moment, I've created a dummy user account but they need to logout of their own account then login as the dummy user, which isn't ideal. I'd like to add a button that's only viewable to Staff users which would allow them to quickly switch user type and allow them to view the site as the dummy user would see it (similar to the "view as person" option on Facebook)? Presumably I'd need to pass different session variables in when the button is clicked, but just not sure how I would do this?