I use Johnson's baby shampoo and it seems to work a treat! I begin by putting some in my hand, running the brush under luke-warm water and swirling the brush around in my palm. When the water begins to run clear I give the brush one last rinse before lightly squeezing out any excess.
In order for the brushes to not loose their shape I find it best to balance them off your desk so you don't have one squished side.
And there we go! Clean brushes which if done in the evening should be ready for you to use the next day.