Back to the Drawing Board: to start something again from beginning because the first plan failed
Example: Your plan doesn't look to be work. Back to the drawing board.
Backseat Driver: someone who tries to establish and maintain control over every situation
Example: It is expected that a former president will be a backseat driver
Beat a Dead Horse: to continue is a totally waste of time and effort as the outcome is already decided.
Example: He keeps trying to pursue them but I think he's beating a dead horse.
Best (or greatest) thing since sliced bread: something is the best and most useful innovation or development invented for a long time.
Example: We think that she is the greatest thing since sliced bread.
Best of Both Worlds: All the advantages from two different or opposed ways at the same time
Example: Works in the city and lives in the country is the best of both worlds.
Bite Your Tongue: to avoid talking that you would really like to say
Example: I should tell Brown "stop it", but I bit my tongue and just sat there.