A guy named Shawn Achor has been studying happiness and how it improves productivity. He has a pretty amusing ted talk. But he also has this 3 minute 5 step guide to being happier, and it's worked for me surprisingly well. He says that as animals we are programmed to look for stressors in our environment, which translates as always looking for something to complain about, and we have limited mental energy, so we often accidentally use up all our brain juice noting all the things that aren't great and then don't get around to noting the good things. So he encourages reversing that, focusing on things that you're grateful for, and reliving good moments so that you get your brain looking for other good things.
5 daily steps:
1. 3 things you are grateful for
2. 5 minutes writing about a positive experience from the last 24 hours in full sensory detail
3. get some exercise
4. stop for 2 minutes and just focus on your breath going in and out
5. 1 random kind act (like an email to a friend, family member or coworker appreciating them)
it's been working for me noticeably.