Put the brush in a container of distilled water. After 5-10 minutes, squeeze the brush a couple of times, then flick out most of the rest of the water a couple (or three) times. Stick the brush on the soap and swirl and plunge for a minute.Transfer to bowl. It should be sticky - tacky. Maybe add a smidgen (half a snurdle) of cream and/or 3 drops glycerin. Now, add 3-4 drops distilled water. Swirl, plunge, beat. Continue adding 3-4 drops distilled water and swirling/plunging/beating until, when you lift the brush, the "peak" curls and collapses on itself. Lift the brush. It should have a shiny lather on it. Apply to face in a painting motion.
For me, this works every time, with any soap.