Use target="_blank" very, very sparingly. You should really only use it to prevent users from losing their place in a site or when they need to reference material while looking at the current page. It can annoy users if every link opens in a new tab when they have thirty-seven tabs open just for your site, and it also short-circuits the back button so that they can’t go back (they’re in a new window with no history, so there’s nothing to go back to in this window/tab).
Getting Braaains Should Be Fun