Always make sure you clean the paint brush after every paint job.
Wipe off any excess paint using a soft cloth. It is always best to squeeze the paint out from bristles starting from the ferrule edge.
Rinse the paint brush in thinner or turpentine oil. If you it is a water based paint use lukewarm water. Never use hot water. That will make the ferrule to expand and loosen up the filament to come out of the brush very early.
If still the paint persists, wash the paint brush gently using a little bit of mild soap. Leave brush to dry at room temperature. Do don’t rest the paint brush to stand on its head down position while drying. Making it will ruin the bristle shape for good. Standing it on the back of the handle is the right position.