Makeup brushes are a breeding ground for bacteria, which is why they need to be cleaned often. This Sigma [+] brush set (comparable to MAC) was purchased 3 years ago on Amazon and it withstands regular cleaning pretty well using the same products I use on my hair. This method can be applied to any brand of brushes.
Fill cup with warm water, making sure to fill below where the ferrule of the brushes begin. Add one drop of tea tree oil and shampoo. Place brushes in the waster and agitate to loosen makeup in brushes.
Add a drop of shampoo on a flat surface. Quickly dip the tip of the brush in the shampoo. Using short strokes, motion the brush left to right as if your are painting. Do this for each brush. This contact cleaning is an additional measure in ridding the makeup deep inside the hair bundle.
After conditioning, follow with an additional rinse of cold water. Squeeze excess water from the hair bundle, making sure to also shape the bristles in the form you would like them to dry. Hang at the edge of a surface and allow to air dry.