I've always washed the brushes well in the spirit (white spirit, turpentine, whatever) by squishing them up and down and against the side of the can until they are clean. I then wash them out thoroughly in a large amount of washing up detergent and warm water. Stand or hang them bristles downwards somewhere warm where they'll dry and be fresh for the next morning. I can make a paintbrush last over 10 years that way but it can take the bristles out if you're too enthusiastic with the squishing. It's easier with paint because you can see when it's becoming clean but varnish is difficult. You have to use your judgement.

LTS