When taking an exam like the IELTS it helps to spruce up your speech by dropping some idioms.

Suggested idioms for IELTS

A situation in which you can enjoy the advantages of two very different things at the same time. 
Example: Working from home gives you the best of both worlds (Useful in Part 3)

Very rarely, not very often.
Example: I only eat out once in a blue moon. (Useful in Part 1 when talking about habits)

Something that someone likes or enjoys.
Example: Reading detective novels is my cup of tea. (This could be a particularly useful phrase in Speaking Part 1, when discussing your likes or dislikes)

A controversial or challenging subject that no one wants to talk about or is difficult to deal with.
Example: Climate change has become a bit of a hot potato. (This could be helpful in Speaking Part 3)

To avoid supporting a particular side in a discussion or argument. 
Example: I sit on the fence when it comes to reducing private cars in cities. (This could also be useful in Speaking Part 3)

To make an argument or bad situation worse.
Example: Deforestation has led to more forest fires, to add fuel to fire many areas are also experiencing droughts. (This could be useful in Speaking Part 3 or even Part 2)

To annoy someone
Example: Litter really drives me up the wall. (This could be useful in Speaking Part 1, 2 and 3 to describe something you dislike)

To have a great time or to enjoy yourself.
Example: I usually have a whale of a time at family gatherings. (This could be useful in Speaking Part 1 and 2).