Watercolor brushes are much more delicate and expensive than brushes used for other types of painting, so a little extra care is needed to maintain their usefulness. The most important thing to remember is to never store a wet brush upright, with the brush end up. Moisture can soak downward, into the handle, and cause the handle to expand. That in turn will loosen the ferrule and cause the brush to shed hairs, peel the paint off the handle, rust the ferrule, and shorten the usable life of the brush. Only store brushes upside down after they have completely dried.
I store my brushes according to use. I prefer long handled brushes when painting oils in the studio, but I use short handled brushes when I paint en plein air because they take up less space in my pochade. My watercolor brushes are stored in a separate vase, and I keep protective tubes on the bristles when they're not in use, and also when I'm traveling with them.
To keep my brushes in perfect condition, I rest them on a sponge to dy them, with the handle end elevated. The sponge wicks the moisture out of the brush, away from the handle and the bristles and prolongs the life of the brush. You should never let brushes sit in water or any cleaning solution because you never want to let moisture soak up deep into the ferrule or into the handle. With proper care, you can expect some brushes to last for several hundred paintings!