I decided to wash my makeup brushes last week, (which was kinda long overdue). I used washing detergent with a little bit of olive oil (that I bought from the chemist), and the results turned out great!!
Now I don't spend too much on my makeup brushes so sometimes the quality of the brush isn't the best, but after they were dry, the brushes were so soft!! Infact they were much softer than when I originally bought them, and it felt so good against my skin when I used it!!