acting like a headless chicken - behaving in a disorganised manner without thinking carefully
bark up the wrong tree - follow the wrong course of action on the basis of misunderstood information
be in hot water - be in trouble
the best thing since sliced bread - something new and considered very good. (It can also be used in an ironic way.)
bury the hatchet - after a quarrel with someone, agree to forget what the disagreement was about
by the skin of your teeth - if you just manage to do something, but nearly fail
to cost an arm and a leg - to be very expensive
curiosity killed the cat - someone might get into trouble if they try and find out about other people's business
don't count your chickens -don't rely on future plans (This is short for : don't count your chickens until they are hatched".)
don't judge a book by its cover - first impressions can be wrong
egg on one's face - someone is humiliated by something they have said or done
face the music - accept responsibility for something that has gone wrong, and be prepared for criticism or punishment
go the extra mile - make a special effort to do or complete something
grinning like a Cheshire cat - having a big smile
icing on the cake - a good situation or activity is made better by something extra.
kill two birds with one stone - achieve two things with one action
leave no stone unturned - do all you can to find something
let sleeping dogs lie - do not interfere with a situation, because this could cause problems
like the back of your hand - know somewhere very well
missed the boat - missed an opportunity
no point crying over spilt milk - it is no use worrying about something that has gone wrong
once in a blue moon - not very often
a penny for your thoughts - what you say if you want to know what someone is thinking about
pain in the neck - very annoying
a piece of cake - very easy to do
pigs might fly - something is very unlikely to happen
put all your eggs in one basket - put all your efforts in one direction with no alternative in the case of failure
no room to swing a cat - there is very little space
sitting on the fence - undecided 
speak of thedevil! - said if someone arrives unexpectedly while being talked about
stolen your thunder - someone does something better than you, and so gets the attention that you deserved
take it with a pinch of salt - the information given might not be accurate