"It ain't over 'til the fat lady sings" is a reference to opera.  Since Americans don't watch a lot of opera, we've stereotyped all opera into being like Wagner's Der Ring des Nibelungen, with the "fat lady" being Brünnhilde.

There was a popular baseball player named Yogi Berra who was famous for corrupting popular sayings.  He took this saying and turned it into "It ain't over 'til it's over", which you may occasionally hear in normal conversation.  (Incidentally, this man's name was the inspiration for the cartoon character "Yogi Bear", although Yogi Bear's personality was inspired by Art Carney's "Ed Norton" character from the television show "The Honeymooners".)

I don't think this one is widely used, but its one of my favorites :- "Ain't nothin' to it but to do it." It means - Don't overcomplicate the task at hand, just go ahead and do it. I took it from Ronnie Coleman, a former IFBB professional bodybuilder. I believe he ssaid it just as he was about to deadlift 800lbs. :D

I think I meant  popular sayings, more than expressions

Couch potato : Someone who spends most of his/her time sitting on the couch, watching TV. 

Here is a site of many 

http://www.infosquares.com/americanslang/index.html

Got it?