Buy brush cleaner. Soak those brushes people! Even just soap and water won't do the trick fully. You need a full out brush cleaner or if you can't run down to the store to buy some, 70% or 100% alcohol will work in a pinch.
A sephora artist told me to spray my brushes with alcohol after each use if I don't wash them everyday (like I have time for that).
I'm more afraid of the brushes the sephora and counter artists use after reading this. I know they clean them but on a busy day, people are lined up waiting. How clean can they get them between customers?