When it comes to training yourself to be open-minded about music, it helps to avoid knowing too much about a band, before you give them a listen, so you can't form too many assumptions before you actually hear the music. It's even more effective if you have no idea who you are listening to. This isn't always very easy, so sometimes you have to forcibly forget your preconceptions about a band, and just sit down and listen to it with your full attention.
(Reading over that, it doesn't actually seem all that helpful, but I will post it anyway.)