very rarely
once in a blue moon
really like sweets and sugary food
have a sweet tooth
memorize something so well that you can recall it exactly without looking
learn something by heart
feel extremely happy
be on top of the world
be very expensive
cost an arm and a leg
feel ill or not well
feel under the weather
go to bed
hit the sack
stop yourself from saying something
bite your tongue
begin studying hard
hit the books