I have learned the hard way how to take care of your art supplies. They are so expensive that you really can't ignore this part of being an artist! There are signs that show if you've been neglectful, or abusive to your tools of the trade (not to mention the emotional trauma to them!)
Does your brush look a bit ragged, with frizzy hair and a bad hair cut?
Is it loosing it's glorious locks of hair one strand at a time?
Is it starting to wobble on its' ferrule?
Is it feeling a bit flaky? (i.e. the paint coming off of its' handle?)
These are all signs of insufficient care and feeding. The fix is simple.
NEVER leave your brush standing in water for several hours....wood is like a straw and will suck it up into the handle where the wood will swell and distort, causing peeling and flaking and gradual deterioration.
NEVER let your paint dry in the brush. The paint will build up at the base of the ferrule and eventually cause the brush to split hairs, not keeping its' shape. (Though if you do have brushes like this, DON'T THROW THEM AWAY! I have found good uses for them in painting foliage and textures that don't need a neat stroke.)
NEVER dry your brushes standing up in a container. The water will soak back down into the ferrule, again causing handle damage. Instead, clean them well (I use a vegetable based soap for both oils and acrylics) shape them and lay them flat to dry. Then I can store them upright.
And yes, occasionally I even condition the brushes! The natural hair brushes have similar needs to our own hair, and need conditioner to restore the natural oils. I have used my own hair conditioners on them to tame those 'frizzies', letting them sit overnight before rinsing them out.