Clean a brush immediately after painting. Use a thinner or solvent that is specially made for the particular kind of paint you have been using. As you clean it, work the bristles between your fingers all the way to the heel of the handle to remove all paint residue.
Thoroughly wash the brush with warm water and soap. Most brushes, particularly those used with water-based paints are very easy to clean.
After washing, comb through the bristles thoroughly. Make sure that even the inner bristles get straightened so the brush will continue to paint properly.
Proper storage is very important in maintaining the condition of the brush. It's best to leave the tip of the brush suspended in a can of solvent overnight. However, make sure the bristles do not rest on the bottom of the can.
For long-term storage, wrap brushes in tin foil or heavy paper after cleaning. Make sure that the bristles are completely dry before doing so.
To salvage a brush with old, dried paint, soak it in a paint-remover type of brush softener. Scrape off any caked-on surface paint, making sure not to break the bristles off in the process. Then wash it, and comb through the bristles as before.