After extended daily use you might notice that your badger brush becomes less effective at working up a good tight lather. You could also notice the the bristles becoming discoloured (especially close to the handle). It could be time to give your brush a clean. As with most things it's always a good idea to keep it clean. I normally clean my brushes every 3-4 months, but it depends on your personal usage and preference.
I recommend using pet shampoo. Badger brushes, as the name suggests are made from badger hair so the effect of the pet shampoo should be just as good as when you give dear old Rover a bath. Wet your brush and leave it to soak in a mug of warm water for a few minutes. Then squeeze a drop of pet shampoo into the palm of your hand, shake off the excess water from the brush and start lathering vigorously. Work up a good lather in the palm of your hand ensuring that the shampoo is soaked deep into the brush. Rinse off the brush thoroughly , and repeat the process.
Finally, leave your brush to soak for a few minutes, then shake off the the excess water and leave to dry. Never put a wet brush into closed cabinet. The result will be a good clean fresh brush.