I think we all can be a little lazy when it comes to washing our make up brushes, I know I can be. I tend to give my brushes a deep clean every two weeks, if I remember, I know I'm bad. But I do remember to give them an on the spot clean every other day or so.
I've went through a few spot cleansers but nothing has come up as good as MAC Brush Cleanser. You may end up wasting some cotton pads by using this, but try using an old muslin cloth. Pour a little of this onto the pad, swirl your brush around and it's clean again. It's perfect for eyeshadow brushes, as you can change colour so many times and it's impossible to have unlimited blending brushes. I've been using this mostly for my foundation buffing brush, which is a brush I use every single day. As I use it every day, I bet you can imagine it becomes pretty gross if you keep putting that onto your face. Just think of the bacteria. So that's why it is important to at least give them a spot clean every day or so.
I've tried a few others, like the one from Sephora and a cheap one from Boots, but this doesn't break the bank and it works. It's only £10 for 233ml and it lasts forever. What more could you want? For a deep cleanse, I use a Dr Bronners soap, which I have in multiple scents but I usually go for the unscented version for my brushes.
What do you use for a spot clean?