After you have cleaned your paintbrush, just rub a tiny dab of hair conditioner in the palm of your hand and apply it to the clean bristles, rinse just a little, spin off the excess water by twirling the brush handle back and forth between your palms, then hang the paintbrush downward to dry naturally. It works great to keep the bristles supple, and less paint gets gummed up into the Farrel of the brush the next time you use it.
I keep a bottle of really cheap conditioner near my utility sink for just this purpose. I would imagine that fabric softener would also work. I use this tip on brushes used with water-based paints. Not sure though if this is suitable for brushes used with solvent-based materials.