imaginaryhuman
First clean them with a solvent like mineral spirits, making sure you get out as much paint as possible. Use a wire brush to get out what you can't get out with a rag. Then wash the brushes thoroughly in warm water with soap (Dawn dish detergent is a good soap, because it cuts through oil). Lay them flat and allow to dry. There are some brush conditioners out there but they don't seem to do any better. There really is no perfect way to clean them they always seem to get worn out or stiff over time.

Fe
Dip them in turpinoid and then wash with soap. Thats what I use and my brushes have lasted for a while.

summerhill
soap powder in the palm of your hand , and work them into the brush and rinse and keep repeating until clean.dry on tissue paper ,shape end of brush and store standing with bristles upwards,