Being more positive about life really isn't that difficult, and with a little bit of effort, one can greatly increase the quality of life.

The impact is pretty powerful, meaning that the more you do it, the more it becomes a part of you. You actually do feel happier.

After having had to stand the whole hour in the crowded train to my work and feeling angry about life, I was pleased to discover this article which describes the following simple five steps:

1. Come up with a positive response to every situation you meet.
2. Look for the good in other people. Rather than seeking to identify the negative traits in the people around you that you interact with, look for the positive ones.
3. Act happy, even if it's a painted dayglow smile. You don't have to be happy - often, that's an impossibly tall order. Instead, just act happy.
4. Drop the sarcasm. Sarcasm can be a lot of fun, but in the end, it's just negativity wrapped up and packaged as a joke.
5. Get plenty of rest and eat a good diet. This along with exercise is one sure way to naturally elevate your mood.

Reference: Benefits of a positive attitude.