This is what my brushes look like after a couple of weeks of using them. This is when it gets time for a little bath and the brushes go all "Yay, yay, bath time!!" and I will then take them for a bathroom stroll.
 Now I take a little shampoo or soap and rub this into the brushes while holding them under the water brush side down. I continue doing this until the water is clear. Never ever let the water run into the handle of the brush (which would happen if you washed them brush side up) as this will ruin it irreversibly and the hair will start falling out.
 Now I get a towel (mine is a special micro fibre one which never really gets wet so the brushes dry perfectly on it) and brush the excess water of the make up brushes onto the towel. Usually there is quite a lot coming out onto the towel which is why micro fibre is ideal.
 Now I simply leave the brushes to dry overnight. I usually clean mine in the morning after applying the make up and the next morning they are all fresh and dry and ready to be used again.
 Never leave your brushes to dry like this as the water may either run into the handle which is just as bad as washing them brush side up or the hair of the brushes my bend into odd directions which obviously also ruins them. Always leave them to dry lying down on a towel. Done!