When you train your mind to think hopefully and optimistically, you will have a fresher perspective on your life and be able to think more realistically. You might just find that what you originally thought was a realistic view of your life was actually a very pessimistic and negative one.
Speak hopefully about everything.
Talk about your job, your health, your future etc., optimistically. Practice this for a week, and then allow yourself a day or two of realism.
Spend time with friends that are positive thinkers.
Make a list of all your friends and identify the most positive ones. Surround yourself with these people and you will find it easier to be positive too!
Counter the negativity of others with positivity.
If someone expresses a negative opinion, you should express a positive one. However, refrain from getting into arguments over it.