For my soaps in mugs, sometimes I'll fill the mug (with soap cake in it) with hot water and let my brush soak in that for a few mins. The hot water soaks into the soap a little and makes the surface much softer. After that, dump out the water, get your brush ready and whip up a lather. Be careful though, if I soak the soap for to long and use a stiffer bristled brush, like a boars hair, the resulting lather can endup on the frothy side instead of being nice and creamy.