Here is what I do. I don't claim to be a pro, but this works for me:

First I clean the container with thinner and a rag. I use plastic containers like cottage cheese cups. Then I put clean thinner in, about 1/2" in the bottom. I clean the brush in that, working it into the brush and shaking it out, usually onto the floor in a big mess, or outside. Have good ventilation. I repeat this process, using fresh thinner each time, until I can no longer get dirty thinner off the brush. It takes 6 to 8 times. The only good brush I have is several years old.

I have read, and tried this, too, but eventually it dried anyway: Not to dry the brush, rather hang it such that the tips float in thinner not touching the bottom. The brush wont warp this way. I had trouble capping the jar so it could not evaporate, but it seemed to work. Perhaps a taller jar with it's lid would work.

Careful with that rag, it'll start a fire if you let it.

Also, I have been successful regenerating a stiff brush using paint remover.

> It is varnish time again. What I want to know is what is the best way to
> clean my varnish brush? Every time I buy a good brush to varnish with,
> after a few cleanings it starts to stiffen up to the point that is hard to
> use.