There is nothing more harmful to the practice of martial arts than approaching a class with the wrong attitude. When you enter the dojo without the proper mindset, the result is often a focus on a particular technique over the art as a whole or, worse, a focus on attaining the next belt or rank.
This focus on attaining a physical representation of one's knowledge is, in particular, undesirable. If one concentrates on a material thing, rather than improvement of one's art, the art itself becomes flawed, and any black belt achieved by this concentration will also be flawed. Such a person ceases to pay attention to lessons already "mastered," meaning those areas that have already been visited will stagnate. A black belt who performs the basics of an art at the same level as he did when he was a blue belt has stagnated. By shifting focus, he may begin to grow again, but every day one follows a pattern makes it that much more difficult to break.
If, on the other hand, one focuses on a constant improvement of the art, the belt or rank will, by nature, follow. It will be unimportant, but it will be there. By focusing one's efforts in such a way, the art will continue to grow. As long as the art continues to grow, it continues to reach for perfection. This is what should be sought in the practicing of a martial art.
As with any change, or any attempt to adopt a particular lifestyle, this is not something that you merely decide to do. Much of this attitude is subconsciously adopted over time. A good master will nudge her students in the right direction, and will weed out any who refuse to adopt the proper mindset. However, as capitalism changes the way that dojos are run, students gain more power in the dojo.
One thing you should think about, and change if necessary, is the way you act in the dojo. Even if it is being learned as a hobby or diversion, the martial art should be taken seriously, or should not be taken at all. Leave horseplay at the dojo door. Joking is acceptable only during times that the sensei is not directly teaching, and only as long as it does not interfere with any of the students' learning (even yours). Make every attempt to focus on whatever you are learning as if you are learning it for the first time.
It is far easier to write these things than it is to accomplish them, and yet it is something you should strive for. It is not something that will be accomplished overnight, nor is it likely that you will even notice it happening. Still, the effect it has on the mind, body, and soul is profound and well worth the effort.