I'm waking up this old thread.
"Put a sock in it!" - a rude way to tell someone to be quiet. It came from back when people used to listen to music on phonographs. They didn't have volume control or headphones back then, so they'd stuff the funnel part of the phonograph with a sock to muffle the sound if it was too loud.
"A piece of cake" - when something is easy to do. This may have come from when people had dances known as "cakewalks", where pieces of cake were given out as prizes, but I like to think it's because cake is something that I usually don't have a hard time eating.
"Kick the habit" - to give up a bad habit. This saying's origin's may have come from people in withdrawal from heroine addiction, as one of the symptoms is restless legs.
"Don't cry over spilled milk" - to not get upset over something not worth being upset about. Back when farmers milked their cows by hand there was always a chance the milk would get spilled. But since nothing could be done about that, there was no sense crying over it.
"Let the cat out of the bag" - to give away a secret. Some people think this came from the old days when people at markets would try to trick someone into buying what they thought was a piglet in a bag when it was really a cat, but if the cat escaped before the seller could do so, then their dirty little attempt at swindling would be out with it. However, I doubt most people, no matte how dumb they are, would fall for this because there would be very un-pig-like sounds coming from inside the bag. Another theory is the cat-of-nine-tails used to torture people used to kept in a bag, and when it was taken out there was no way out of it. 