Too much of a good thing (As You Like It) – used to warn against excess. Too much of a good thing can be bad for you.
To wear your heart on your sleeve (Othello) – meaning to show your emotions, especially romantic feelings, openly.
There’s method in my madness (Hamlet) – meaning that beneath apparently chaotic behaviour there is some kind of order.
The green-eyed monster (Othello) – meaning jealousy (jealousy and envy are associated with the colour green in English, as in ‘green with envy’).
The world’s your oyster (The Merry Wives of Windsor) – this means being able to do anything you want because you have the opportunity to do so, either through talent, looks, luck, power or money.
To make your hair stand on end (Hamlet) – used to describe something very scary.
They’re all still in common use today, so why not try throwing one or two into your conversation?