Since I began wearing make-up properly I have always relied on Johnson's Baby Shampoo to
deep cleanse my brushes mainly because it was cheap, cheerful and accessible. 
However sometimes I found that I have to re-wash my brushes a few times as they aren't as clean as I would like as well as being a pain in the neck it is rather impractical so after asking what others use
to ensure everything is squeaky clean Dr Bronner's Magic Soap cropped up time and time again.

Dr Bronner's Magic Soap comes in lots of variations but the one I favour is the 18-in-1 Tea Tree 
Pure-Castile Soap which is constructed using organic oils, my reason for vouching for the Tea Tree
variant is that it is a natural spot busting ingredient and having oil, blemish prone skin I figure
somewhere so how down the line it will help my skin...i'm so logical haha!

If like myself you are really cautious with your make-up brushes and why not they are far from cheap
then you really must pick up a bottle of Dr Bronner's Magic Soap at some point as it gently cleanses all brushes, dissolving any means of dirt, grime, make-up and oil without stripping the brushes 
or drying them out, which will ruin them eventually.
You only need the smallest amount of soap to deep cleanse any brush including those extra mucky foundation or stipple brushes, massage in, rinse and watch the make-up disappear in a jiffy,
it really is rather satisfying in a bizarre way.

Of course you can use the soap for a whole multitude of things including brushing your teeth 
but for me this is one of the best make-up brush cleansers out there!

Dr Bronner's Magic Soap  18-in-1 Tea Tree Pure-Castile Soap* £8.50/237ml via ASOS - link