Smile – it is the best and the easiest way. Just try to smile in front of a mirror; it really helps to change the mood and will also help to relieve the stress.
Meditation – try to meditate and divert your thoughts. It is a very relaxing process, which helps to ease your mind.
Be with positive people – make friends who give you constructive but still a loving feedback. When you get some negative thoughts, then try to talk to people who have a positive attitude; they will help in setting the things right.
Sing – singing makes you feel better, even if you don't know the correct lyrics of a song. Singing helps to bring out feelings and feel better.
Reading positive quotes – collect some positive thoughts that you like and paste it on your computer, fridge, door, etc. It will help you to remind those thoughts.
Help someone – doing something good will help in diverting your mind and you will feel better by doing such things.
It is very difficult to be positive when negativity is surrounding you, but in such situations you should know that attitude is something which is under your control. All the negativity in the world will not be able to bring you down if you are not allowing it to get inside your mind. So, try to defend the negativity in your attitude and channel your way towards positive attitude.