Condition them with oil. Some people like using extra virgin olive oil; I like using argan oil. Either will do. Whichever oil you choose, what you're going to do is wet the bristles, then swirl them through a few drops of your oil for a few seconds. Rinse the brush out, then wash with anything BUT your brush cleanser (baby shampoo or Dr. Bronner's liquid soap are popular choices). Rinse, squeeze the excess water out, then either lay your brush flat to dry or hang it upside down (the latter is more ideal because then the moisture will be drawn down by gravity and won't get into the ferrule. I definitely recommend getting a Benjabelle brush tree of some kind). Once your brush is dry, it should be nice and soft. I don't remember the ingredients, but I think MAC's brush cleaner has alcohol of some kind in it. It's not a harmful ingredient per se, but it can dry out a brush's bristles and make them feel scratchy against the skin. Deep cleaning and conditioning every so often will help your brushes' longevity.
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