You might try putting them in curlers, so to speak. You do this after washing the brush, but while it's still wet.
This is as simple as folding a stiff bit of card, such as a business card, tucking the bristles toward the fold, then clamping the card. The clamp is a bit tricky - too stiff a spring and you splay the bristles quite a bit, though that doesn't always make for a bad tool once it's dry. When I do this, I tend to use cheap Chinese spring clothespins. Larger brushes require 2 or more of the clothespins, as the bearing surface is pretty narrow.
I've also experimented successfully with (to carry on the hairdresser analogy) a bit of perm liquid... very dilute water-mixable linseed oil. Too much linseed in the solution and you just get a stiff and mostly unworkable brush.* Thinned out adequately, the dry-ish linseed makes for a firm, knife edge.
*Savable with a cleaning with Master's Brush Cleaner or, if you're cheap like me, Murphy's Oil Soap.