Keep your brushes clean throughout your painting day without the use of solvents.
To keep my brushes clean throughout the day I use walnut oil in a Silicoil Jar.
First I wipe all the paint I can possibly remove onto the lint free blue paper towel depicted on the bottom.
Next, I swish the brush around a bit and then wipe it on the paper towel. A lot more paint comes off quickly this way. I do this a couple of times to minimize how much paint ends up in the jar.
Then once I'm satisfied with the level of cleanliness of the brush, I squeeze absolutely all the oil out of the brush with a clean part of the paper towel to ensure I'm not adding excess oil to my painting.
The Silicoil is great because of the smooth metal wire inside that gives your bristles something to work against, while allowing the pigments to slowly fall to the bottom. I replace my oil about once per year so the cost of the oil is pretty minimal. I use M. Graham Walnut oil because I feel confident that if some of the oil in the bristles does get into the painting that it is of a high quality and will not cause problems with the paint film.