Thoroughly rinsing your brushes with clean water when done painting for the day is semi-sufficient. It is a good thing to clean your watercolor painting brushes occasionally with soap. It helps condition the bristles and clean out some of the hidden paint. I use the Master's Brush Cleaner and Preserver. It comes in a little round container and so far it has lasted me a very long time.
1. Gather your brushes, the brush soap and dip your brushes in water.
2. Swish the brush over the soap a few times.
3. I then swirl the brush on my white painting table, to lather up the bristles. Rinse in water and repeat. I repeat this process until the soap lather is white and the brush is clean.
4. Rinse in water. I use two water containers. One that is for the first soapy rinse and a second container of water for a cleaner final rinse. Then with your fingers, carefully form the bristles to a point and allow to dry flat.
This is a simple process that does not take long to do and it will help extend the life of your brushes.
Here's an extra little tip: Have a brush with a stray hair or two? Use this brush cleaner to clean your brush. Then lightly rinse the brush leaving a very small amount of soap behind. Using your fingers shape the brush to it's original point or shape. Allow to dry completely. Just remember to rinse the brush well before you begin to paint with it to remove all the soap.