I was chatting to a makeup artist at the weekend(she does weddings etc),and uses Fairy Liquid.
You need to wash once a month for powder product brushes and wet(concealer, foundation any liquids), after each use.
I have a spray Paula Dorf which I use inbetween for the brushes used with powder products. I spray on kitchen roll and wipe.
I use the proper cleanser with by Beauty Blender and even use it for cleaning other brushes, or just use soap(I bulk buy from the christmas market fresh vegtal soaps), so usually shea butter.
Hold the brush handle down, do not get water up into the part where the bristles are glue in which will fall apart and bristles far out. Warm water and massage the wet bristles with those cleanser and rinse under the tap until the water runs clear.
I admit to buying Japonese(sp) goats milk brush soap, it comes in a little pot and you damp the brush and run over like solid shaving soap. A little does the job.