It's often said 'you shouldn't smile till after the christmas holidays', however that's not necessary to maintain a good atmosphere in school or the classroom. How else do you show your personality in the classroom, if you don't smile. I would also go a step further and say maintain your humour throughout all aspects of your life otherwise it can all become too serious.
When you make a mistake or don't think you have done things as well as you wanted, find the humour or the fun in it all. Rather than ruminating over anything that doesn't go well or anything you say which isn't great, find the humour in it or in a next moment and move on.
Allow humour in the classroom and laugh with the students and you will also be teaching them to see the lighter part of life. When we are more in the present moment, we will be able to appreciate all about that moment and experience the joy and humour within each situation. Allow the fun in and embrace the lightness in interactions.
This will come naturally over time as you get used to your students and it may be necessary just to reiterate the ground rules so it doesn't become laughing at each other, however that can easily be created from the start as you model humour, the power of humour and how you can all use it together.
This post follows on from the 'Be present' post as I believe it is a natural advantage of taking time to be fully present as we can see all there is in each moment and all the opportunities it holds. Laughter is a great form of medicine for us all and it allows us to let a moment go by rather than adding it to a list of things we aren't happy about.