All you need is conditioner. Doesn't everyone have some lying around somewhere? Cause' I know I ALWAYS do. Oh and you probably need your positive cleaning mood on.
Then grab the brushes that need cleaning.
Squeeze out some conditioner onto your clean hand and wet the brush.
Using circular motions, rub the brush onto your hand and mix it with the conditioner- you will probably see all the disgusting old foundation that has been sitting in the brush for a while- yuck.
You want to do this for about a good three minutes until the brush is completely clean.
Lastly just wash out the excess conditioner and rub the brush on a towel- when you leave it out to dry over night, do not leave it standing up as this will ruin the bristles. Instead, just leave it flat against a towel or the best way, leave it hanging upside down.