Keep the bristles of the brush soaking in mineral spirits when not in use during your work session. The mineral spirits thin whatever residue of paint is left in the brush; this prevents the brush from drying and hardening. If you leave it soaking longer than your work session, the bristles can bend, and residue in mineral spirits may drop to the bottom of the container and settle in the brush.
Wipe the brush off on a rag to remove paint residue when the work for your session is complete. If you still see paint, swish it again in the mineral spirits and wipe it again on a rag.
Wash the badger bristle brush with soap under warm, running water. This removes the mineral spirits and any other remaining residue. Soap the brush gently. Stroke the bristles downward across the fingers of your open hand under the water. Do this in one direction; otherwise, you may drive residue into the ferrule, the metal that crimps the bristles. Rinse thoroughly.
Shake out the remaining water. Store the brush upright in an open container. Any pressure on the bristles while they are drying can bend them.