When I arrive home from work, the very first thing I tell my wife is the best thing that happened that day. No exceptions. No complaining. Just the best thing that day, even if it was just a good cup of coffee. This had the effect of starting our evening off on a positive note and it changed our relationship.
"assume positive intent". All this means is that when someone does something that upsets you -- like cuts you off in traffic -- assume they're just trying to get through their day and in no way, shape or form were targeting you. There was no malice, just an awkward move in a fast-moving world. This little nugget has helped me immensely.
Move closer to where you work -- even if the home you can afford is smaller. Studies have shown that a small home near your work will tend to make you happier than a large home with a long commute. Not only is a bigger house more work to clean and maintain, you have less time to do it because of all the commuting.
Don't compare yourself to others! The conditions that brought about their achievements are vastly different than yours, no matter how similar as you two may seem. Focus on your own successes and learn from your own mistakes.
Be very, very careful about which outcomes you attach your happiness to.
make your friends feel good. Thank them, congratulate them, smile at them, celebrate their successes. They'll return the favor, and everybody wins. And don't return calls from people who don't operate this way.
Also, when things go wrong, don't sweat it. Tomorrow is a new day. Few problems are permanent, and most can be fixed quickly. Fix what you can, accept what you can't, and always look forward.