I use Murphy's Wood Soap, which can be found at most grocery stores and olive oil - the most inexpensive olive oil you can find.
I purchase my olive oil Trader's Joes. It is about $6. Most grocery stores have an inexpensive version. This is not the Extra Virgin $30 a bottle stuff.
I place my brushes in the sink. In order to tell which ones I have cleaned I place the unclean ones with the handle facing me. If I have cleaned it then the brush part faces me.
After cleaning each brush I rinse with warm water and then repeat using the olive oil.
My sink looks like this:
I use natural bristle brushes and this process restores some of the natural oils in the brush.
Now, one last step. clean the sink! Remember to use a glove as the paint is a chemical.