This year my happiest moment was when I finished the 70km ultratail in the middle of wonderful mountains. When I arrived I was tired, but I felt like I was the boss of the world! :)
"Plateaus" implies a two-dimensional outlook on life, which is not even how life works. Life isn't even four dimensional. It has many, many dimensions that grow and die out. You can be happy, sad, afraid, and horny all at the same time. I.e. happiness is just an emotion, and you can feel several emotions at the same time.
This is such a primitive outlook that I don't even.
I try to seek out new experiences, social or otherwise.
This past year I started keeping a short diary of semi-significant events - a concert I went to, a date I had, a problem I solved at work. My goal with the diary was to make time slow down (without moving) and it has definitely worked. It's really fun to read back through it.
Have a social life(not refering to facebook or google+). take time for your friends and family. Talk to them regularly and don't lock yourself into any dark room- thats my mantra.
http://www.youtube.com/watch?v=BVDgV7cbNjU