I know. I know. Ex-anythings are rarely spoken well of unless they are dead. And if you've left any of your previous jobs on less than your own terms, it's easy to dish the dis on ex-bosses, ex-companies, ex-co-workers, etc. But now's the time to apply another old adage you might have learned when you were a tiny tot - "if you can't say anything good, don't say anything at all."
Speaking badly of your previous employers during a job interview is the kiss of death. Why? Because employers identify with other employers. What you do unto one is a pretty good indication of what you will do to them so if you complain about your last boss, talk badly about a company you've worked for in the past and or comment about what lazy butts your coworkers tend to be, the game is over. It's the equivalent of hitting the eightball into the corner pocket during a game of pool - doesn't matter how well you played the rest of the game, you've just lost.
The solution? Focus on yourself (and this is the only time I'm giving you this piece of advice - normally doing so is counterproductive). Talk about what you have been able to accomplish in previous positions, give examples of what you have done to help your previous employers, focus on what value you can bring to them. So when you're heading out for an inteview check your negative opinions at the door and leave your baggage at home - you'll soar higher if you travel light.