I know this topic is old, but I was perusing the topics & figured at some point someone else would do the same.
Brushes are your best friend. Buy the most expensive you can afford and take care of them by:
1. Cleaning promptly after use.
2. Complete cleaning after your modeling session. Use a good brush cleaning soap, available at art supply stores. They deep clean and condition the bristles, leaving them soft and pliable for the next time you pick it up.
3. Lastly, DO NOT put a freshly cleaned and still wet brush upright--leave it lying flat. Otherwise, the bristles will eventually rot out at the ferrule (place were bristles attach to metal sheath).
Also, the person that started this topic mentioned that the bristles were "splitting"--this can come from roughing a delicate brush up too much, or perhaps a less expensive brush. You can try leaving some of the brush cleaning soap on the brush after cleaning, shape it to a tip with your fingers, and leave it to dry that way. It might help. I have picked up many brushes, thinking they looked fine with a nice little point--only to have the thing NEVER keep a tip while painting!
It's a battle sometimes, but if you find that "magic" brush, treat it well & it should give years of trusty service!
--daredevil