Extra storage doesn't have to cost much; utilise what you've already got. And it doesn't have to take up much room.

Sliding shelves
Place a bookcase on castors in front of shelves to create a hidden storage compartment behind.

Maximise wardrobe space
You might be surprised how much you can fit into a wardrobe by dividing up the space and compartmentalising everything.

Boxed in
Make a home for your children's favourite toys - and a temporary home for those of visiting kids - with sturdy archive boxes.