It's important to regularly clean your makeup brushes because product and bacteria build up in the bristles and you don't want that buffed into your skin! I wash my brushes about once a week depending on how much I've used it and what I use it for. I mostly use discretion to tell me when my brushes need a cleaning. When I have no more clean, usable eyeliner brushes I'll wash them and so on. While some prefer to buy bottles of Mac Brush Cleanser and some just use good ol' baby shampoo, I use Loreal's Go 360° Clean Cleanser for Sensitive Skin.
I used to use my shampoo (whichever I was using at the moment) to clean my brushes and that worked fine. I bought this cleanser with the intention to use it on my face but it didn't really clean well and did nothing for my acne. It kind of sat around my bathroom for a month and one day I decided to use it to wash my brushes. This actually works pretty darn well at removing all the gunk in the dense bristles since it's supposed to remove makeup on your face. I've always had problems cleaning my eyeliner brushes after using them with Wet N' Wild's gel liner because that stuff dries and STAYS PUT. Amazingly, this easily removes that residue and leaves my brushes good as new :) A little of this stuff goes a long way and it's relatively inexpensive at the drugstore. I think I purchased my bottle for $6. However, I feel like this makes my brushes smell weird after cleaning them. Not sure what kind of smell it is because it doesn't smell bad in the container but after my brushes dry they smell quite bad. If you're sensitive to scents (or I guess lack of) then maybe you wouldn't want to use this to wash your brushes. But I'll deal with it since I have so much to use up.
Although this didn't work out on my face, it does a good job at cleaning my brushes and I'm glad this didn't simply end up in the garbage! I love finding different ways to use a single product, don't you? :)