if using liquid soap, one or two drops in the bathroom basin with no water. swirl the brush in it and pull it back and forth -- then rinse with warm water. do this once a week.
if using bar soap, make sure it's good quality glycerine soap. wet the bar, pull the brush back and forth across the wet soap. rinse as above. always clean your brushes at least once a week.
then drain laying on its side. not with the handle down and bristles up, as that will undo the glue that keeps it in good shape.