Cleaning makeup brushes is something that I have always "forgotten" in my routine. I just didn't think about it. If I thought about it, it was while I was using the brushes or at a time that I knew I'd need to use the brushes shortly thereafter and didn't have time to let them dry. Well, I have tried to turn over a new leaf.
Why, you ask? Because they hold on to oils from your skin. The more you use them, the more build up there is. Plus, wouldn't you want to clean off some of that old foundation build-up too? I would certainly think so. But, if you don't care about any of that, here's another reason: I've ended up with a "custom eye color" on my eyes due to the previous day's eyeshadow. Niiiiiice. So it also cleans residue and provides a clean slate.
It's pretty easy and really doesn't take long to clean brushes. It's almost silly to show the process, but I feel like other step-by-step cleaning instructions I've seen are pretty in depth. Special brush cleansers, hand mitts, soap mixtures, etc. Not for me. I just use good ol' every day shampoo. I mean, it's all hair, right? I used to put some shampoo in the palm of my hand and swirl the brush in it until it lathered and got all the product out (it's pretty amazing how much product stays in the brushes, honestly). Buuuuut that used a TON of shampoo and seemed wasteful. So, I simplified the process.
Get a small bowl and add water and shampoo - warm water works best. Swirl the brushes in the bowl until they're clean. It might take a couple refills depending on how dirty the water becomes, and depending on how many brushes you have. Once they've all been through the shampoo/water mixture, rise all of the shampoo from the bowl and fill it with warm, clean water. Follow the same steps swirling each brush in the clean water to remove excess shampoo.
Place the clean brushes on a towel to dry. I usually try to re-shape the brushes so they dry in their original form. I find that they typically take a few hours to dry all the way (so I wouldn't start this process if you're in a rush or if you need to apply your makeup soon).
That's it! Easy peasy. Now you'll have fresh, clean brushes next time you apply makeup. As for how often this should be done...well, I'm not a good example. I maybe clean my brushes once a month or once every two months. It should probably be done every couple weeks. Just do as I say, not as I do. :)