I purchased a couple of (expensive) Epifanes brushes, years ago. Epifanes recommends rinsing brushes in mineral spirits, rinsing again in clean spirits, then spin them out (the handles are round for a reason). Then store the brushes with the handles vertical and the brush section suspended in the diesel. When it's time to use the brush again, simply give it a rinse in clean mineral spirits, spin it out, and it's ready to go. I stored them in a coffee can, made a galvanized cover with a hole and a support to hold the brushes. I used the storage system for almost 8 yrs. and the brushes stayed like new.
Washing the brushes dries out the bristles and makes them brittle, they lose their brushing function. The diesel method requires extra effort, but does work.
An alternative to that is foam brushes. I switched years ago, I've abandoned the diesel storage method, and now just keep a box of 3" JENN foam brushes in my varnish supplies. The results are equal to bristle brushes, and cleanup is simple. Don't try to use box store foam brushes for finish varnish, you'll be disappointed. Use JENN brushes, it makes a difference. The technique is a bit different, but the results are as good or better.