I don't believe that any particular person could be absolutely and indefinitely open or close-minded, everything in life has its limits. I associate open-mindedness with understanding, compassion and acceptance. Like most things keeping a truly unbiased open-mind can be easier said than done, however I do believe that it is well worth it.
I think that the more you live, the more observe, the more mistakes you make, and the more people you grow to care for, the more your perspective starts to widen. When you find yourself or someone you love in a situation you never expected, you may act or see actions you would never expect. The difference is when the situation is so close to you, you reserve the harsh eye that you may look upon a total stranger with.
When I was younger I had very critical views, I had very strict ideas of what I viewed to be right and wrong and not much sympathy for those who I believed acted wrongly. I was too quick to form judgments of situations and people without knowing many details. I still do have a strong vision of what I think is right and wrong, and I hold myself to those standards before anyone else. But, through life I've learned you can't always expect the same things of everyone, you don't know what people have been through or what they are currently going through. The people with the most going on usually talk about it the least. You really need to learn to not take people's behaviors personally and respond with kindness as much as possible.
Clearly, there is a line. There are things I cannot accept and actions I could never stand for. On the other hand, there are situations I've grown to see other sides of. People hold back so much in spite of judgement, they hold back being themselves, having fun, sharing parts of who they are, and I find that very sad. That's why I personally vow to try my very best to reserve judgment, and keep an open-mind as often as possible.
Also be open-minded when it comes to trying new things, visiting new places, eating new things, trying new hobbies, you never know what you'll absolutely love. The very concept of applying open-mindedness applies to endless things, however the one most beneficial may be people. Never stop yourself from getting to know another person for a frivolous reason, they could end up becoming one of your best friends.
While there is a certain level of respect represented by keeping unkind or judgmental thoughts to yourself, try and take it to that next level and minimize those thoughts in general. Reason with yourself and realize when you're being close-minded, consider all the different factors that go into a situation that you are unaware of.
Maybe try a little more often to put yourself in other peoples shoes. Treat anger you stumble upon with kindness. Hold back a little less yourself, and view the world through a more open-mind, and watch as it becomes a brighter place.