My wife taught me this when we were dating. Sit upside down on the couch and start smiling. In minutes most people we have spoken to that have done this start laughing in less than a minute and often it becomes almost uncontrollable. Cami and I begin laughing and can't stop for several minutes. We're not sure if it's the blood rushing to our brains or the absurdity of what we're doing but it works every time.
Another way we can always get ourselves to laugh is by watching one of our two favorite comedians: Brian Regan and Tim Hawkins. Here is a YouTube clip of each of them. Enjoy!
Remember to laugh together. The couple that laughs together stays together.