Clean your brushes at the end of every painting day, and never leave a brush soaking with the bristles touching the bottom of the container.
First, remove the excess paint on a piece of newspaper, then rinse in white spirits (oil paint) or water (acrylic paint) and wipe with a rag.
Wash under warm running water, soaping the bristles with a bar of household soap.
Rub the soapy bristles in the palm of your hand so that the paint which has collected around the base of the ferrule is loosened.
Rinse in warm water, shake dry, and then smooth the bristles into shape.
Leave the brushes to dry, bristle end up, in a jar. Always make sure brushes are dry before storing them in a closed container, or they may develop mildew
Dettol or normal floor antiseptic household cleaner is also a great tool for getting those hardened paint bits out of your brushes and make them new again. Just leave overnight in a tin or jar, wipe and wash as normal.