Interview this morning..need advice

I'm wanting to leave my current job because I don't like the way things are done there and don't want to work for this particular company because of it..but I'm interviewing this morning at a job similar to the one I already have, just a bit different and different company. When they ask me "why are you wanting to leave your current job?" What am I supposed to say? I've always been told not to bad mouth companies when interviewing but I can't say anything like "it's just not a good fit" because I'm interviewing at a similar place. Can anyone give me some ideas on what I should say?! My interview is in a half an hour.