Whenever I am at an interview I always seem to fall into the trap of being negative about past employers because I have had bad experiences in the past. I know that this doesn't make me sound good, as it's going to make them think I'll say the same about them to my next employer. How can I turn my bad experiences into positive thinking at an interview?

You shouldn't focus on the negative, think about what you learned from it and how you emerged a better person for it. For example, you may have had a dispute with a manager. Think about how this could have improved your problem solving skills and how it made you more convincing when presenting an argument. Don't mention people by name and be aware that the interviewer will be paying close attention to the way you word things.