It almost seems silly to ask if you want to get better as a teacher. If you don't seek improvement, what are you doing? Students change, curriculum modifies, technology grows. If you're not improving with it you are being left behind. I feel contractually obligated to improve and that feels like a healthy thing.
There are lots of ways to seek improvement but one of the best, quickest, and most effective ways is to seek feedback from a trusted colleague. I like the word 'feedback' much better than 'criticism' or 'critique'. Feedback implies that someone watches me teach and then reflects back to me what they saw with some possible ways that I might improve. It has to come from a colleague I trust because I have to know that we have mutual respect. It could come from a complete stranger but I would not know if he had an agenda or not. With a trusted colleague I would know that his only goal was to help me improve.
The feedback also has to be based on standards, not just on personal preferences. That way I have something to measure the feedback against. I don't have to agree with the feedback but I like to know that it's not just your personal feelings. If I give feedback and say "I just didn't like that" that is not enough. My liking or disliking has to be linked to an objective measurable standard. That tells us what the ideal is and then we can measure ourselves against that ideal and work towards improvement. If the standard is "I like it" or "I don't like it" that is just a personal preference.
Even in the best of circumstances the feedback tends to sting a little, but you have to learn to get past that. I once gave feedback to a student teacher. I had three things to tell him, all standards based and all given with love and respect - he knew that we were friends. After I gave him the first piece he had a stunned look on his face. I could see that he was hurt, so I slowed down but his pain was visible. His lip quivered. I realized that he could take no more that day so I thanked him for his efforts - which were good - then said goodbye. The next time I saw him he told me how deeply I had offended him with the previous feedback. I apologized sincerely because I never want to hurt someone in a situation like that. He looked at me very seriously then said "Thank you, I accept your apology." After that was settled I had to tell him the following: "if what I told you that day was so hard to take, then you have absolutely no chance of surviving in this environment because we all give feedback and it is open and honest but it can be painful." He didn't survive.
Once I had an idea for a technique in class that I thought would be very useful. I worked on it in my office then planned to integrate it into a certain class. I invited 2 colleagues to come to the class and observe then give me feedback on that specific thing. During the class I felt like it wasn't working as I thought it might. After class the three of us sat and talked for about an hour. They also thought it missed the mark, but not by much. They both offered some possible tweaks and we all walked out together. As we were walking down the hall I did some quick math and realized that among the 3 of us was over 90 years of teaching experience. We all still felt good about giving and receiving feedback. I integrated the changes that we had all agreed on and tried it again. It worked much better and it was all thanks to honest feedback.
I don't know why teachers are so resistant to receiving feedback. Maybe because to watch a good teacher is to watch something that appears easy to do, but then every talented professional makes his or her skill look easy. Perhaps it is the equipment involved in a building trade that gives it the look of something difficult to penetrate, even though the journeyman makes the work look simple enough. To watch a really good teacher is to watch someone that just appears to be talking - just standing up in front of the class and talking with kids, calling on kids, joking with kids, etc. You think "that's easy, I can talk with kids." Then you try it and find out that there is much more to it than that. But it looks so easy that you can't bear to ask for help, you can't even bring yourself to admit that you might need help. You think "I like kids and I like to talk - I'm a teacher, but it's not working so well and I don't know what to do". The thing to do is to get some feedback.
So get over yourself, gather a little humility and get some help. That's not an admission of weakness, it is a declaration of desire. I want to get better and I am going to find help doing it. It is a great thing. Your students will thank you.