Perhaps others may (or surely) know better, but this is what I usually do anyway:
- Immersion in water with a few drops of dishwasher liquid for 20 mins
- Lather with a nice, cheap and rich shave cream (RSC, ToBS, Tet-A-Tet)
- Leave brush standing on its own for whole night, with the full lather on
Next am the brush is much much softer, smells nice and is kind of 'broken in' (but needs more, of course.)
Some people though say this is not really necessary as the brush will break in after prolonged use anyway.
Perhaps Fido or Sir-Ed8 can advise further.