Good quality makeup brushes can last for many years if properly cared for. I wash my brushes at least once a month and spot-clean them as the need arises.
For spot-cleaning I use Parian Spirit Pro Makeup Brush Cleaner.
It comes with a foldable spout, but I find that transferring small amounts in an atomizer and spraying it on the brush is easier and more economical.
Here's my MAC eyeshadow brush before spot-cleaning:
Here it is after I spray it with Parian Spirit. See the makeup coming off the brush?
Wipe the brush with a tissue until all the makeup comes off.
Here's the brush after spot-cleaning.
You can buy Parian Spirit from Suesh Makeup Toolkit.
When washing your makeup brushes use baby shampoo or a mild liquid soap like Ivory. Pour this on the palm of your hand, add a little water, and swirl the brush on it until clean. Rinse well, squeeze gently and re-shape the brushes to their original form. Lay them on a towel in this position:
Note that the bristles are hanging over the edge. This would ensure they are completely air-dried.
Be careful not to submerge your brushes in water. Doing this will make the bristles come off as they are only held together by glue.
Hope you learned something new =D