There are lots of threads on cleaning brushes. You can use borax, or you can use vinegar. I haven't cleaned mine in 2 years (and I use it every day), and it still works great. But I think I'll clean it today with some borax and water.
Keep it handy while you're watching TV. The moment an ad for a multi-blade shaving system airs place the boar brush close to the TV. We all know that the ad will be pure hogwash. Repeat as necessary.
For the initial cleaning, I use a couple of soak and rinse sessions with dish liquid, followed by lathering with a strongly-scented soap such as Tabac. For maintenance cleaning, shampoo works well, followed by a short soaking in dilute vinegar. Rinse and let dry.
Short soak in diluted vinegar works the best IMO. Gets all the soap scum out of the knot. I usually follow with a lathering of shampoo or dishwashing soap to get the vinegar smell out afterward. I do this every couple months, but I'm sure I could stretch it out longer.