After a while, brushes can become clogged with wax and even dirt. One way to clean a brush is to aggressively rub a scraper edge against it going back and forth. Another way is to take two brushes and put snow between then and rub them back and forth. The snow helps to remove the wax.
Others wash their brushes with very hot water or even put them in the dishwasher. These practices are discouraged because after prolonged exposure to water, the wooden brush handles will warp.