How to act like your laughing?!

How to act like your laughing?

Im doing a sketch this weekend where me and a bunch of my friends have to act like were laughing ALOT, like look at this sketch-
http://www.youtube.com/watch?v=ck-dkbf-t...
well basically we have to laugh like that..so how? i cant without seaming REALLY fake. how do I laugh????? (haha never thought id be asking that =])

once the sketch is completed ill put it up here so you guys can tell me hwat to improvie on, thank you!!!1

thanks