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.
I am right handed so I put a glove on my left hand and put a small amount of Murphy's wood soap in my hand and the wipe the brush through this many times.
After cleaning each brush I rinse with warm water and then repeat using the olive oil.
My sink looks like this:
This two part process gets the brushes clean. Sometimes I do need to repeat the process. Each brush is then individually wiped clean. This is the final test if they are clean. Too much paint showing up on the paper towel and they are not clean.
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.