Never leave brushes with acrylic paint on them to dry.
Acrylic paint is like plastic- once it dries, it is impossible to remove it from a brush. I learned this lesson at the beginning of my first art studio course at MacEwan College. The whole set of my brand new brushes were ruined after the first painting class, because I didn't keep them in water. Being a poor student certainly didn't help!
Once the painting session is finished, make sure to give a good rinse to your brushes with soap and water. After that, tap the tips of your brushes with a clean cotton rag or paper towel to dry them. Finally, bring the hair of your brushes into shape by smoothing them with your fingers and let them rest flat until the next session!
If you notice that your brush hair is very dry, moisturize it by rubbing a touch of linseed oil into its surface. The brush will be softer and easier to work with next time. Pamper your tools!