Most professional decorators I know store their solvent used paintbrushes in a special storage box called a Brushmate, which contains a vapour pad which stops paintbrush bristles from hardening, the Brushmate box and its solvent, might seem expensive at first especially for an occasional painter; however in my opinion they are well worth the money.   The small 4 brush storage box is available for about £15 from many retailers.     Large Brushmate boxes are available which can store about 20 paintbrushes of various sizes these boxes are available for about £60.   

The cheapest and most traditional method to store oil paintbrushes is to place them in a container of clean cold water.  It is important that you only submerge the bristles and not the metal ferrule that binds them as water penetrating the upper stock may cause the ferrule and handle split apart, the paintbrushes will stay soft as long as there is water in the container.

Please note if you do choose to store your paintbrushes in water, always shake any excess water from the paintbrush before resuming your painting and change the water in the container regularly as it can quickly become contaminated and begin to smell.