Exercise, I've found, is probably the single most important aspect of maintaining "happiness."
The problem is I can't consistently muster the willpower to block out an hour long chunk of time to go running or workout at gym. I found a lovely compromise, though:
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The Seven Minute Workout [1][2]
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It removes literally every excuse my brain can come up with not to work out. Too cold outside? Doesn't matter, you're not going anywhere. Not enough time? The time you'll spend coming up with excuses is probably longer than 7 minutes. That's like two songs on the songza workout playlist. Don't feel like changing? Doesn't matter - you can do it in the clothes you're in. Can't find the equipment you need? All you need is a chair, a wall, and a 7x3 clear space of floor.
I figure it's much better to do this every day [3] than to go on a workout binge where I work out 2 hours at the gym on Monday, take Tuesday off, dread going there for 1.5 hours on Wednesday, take Thursday off, take Friday off because I'm still sore from Wednesday, take the weekend off because it's the weekend, and then forget about working out again.
1. http://well.blogs.nytimes.com/2013/05/09/the-scientific-7-mi...
2. http://www.7-min.com/
3. http://dontbreakthechain.com/share/christiangenco/last-four/...