It's vital to the life of your brushes to care for them as their value warrants.  For water-based paints, switch back and forth between rinsingout with tap water and midly soaped water.  For oil and alkyd paints, use mineral spirits or turpentine in the same manner and finish up with mild soap or brush cleaner and water.  Always reshape the brush when work is complete.  Remember that solvents are toxic and should always be disposed of as hazardous waste.