Better quality artist's brushes will last a long time if cared for properly. This article discusses some of the best ways to care for your good oil painting brushes. Caring for acrylic and watercolor brushes is a simpler process. You just rinse thoroughly with clean water, apply a mild soap (Dawn liquid will do), and rinse again. Conditioning with Master's Brush Cleaner is always a nice finish.
Oil painting brush care is a bit more complicated, but certainly worth the extra effort to extend the life of your brushes. I'm going to share a few different ways to clean your brushes and let you decide what works best for you. Timing is a key element in cleaning your oil painting brushes. If you plan to continue painting the next day, then you can simply wipe the paint out with a paper towel or clean cloth, swish in your mineral spirits or odorless thinner like Gamsol, wipe again, then set them away. You can also wipe out excess paint, dip the brush in a light oil with a few drops of clove oil, and lay horizontally and somewhat elevated to use the next day. Just be sure to work again the next day!
One simple strategy to clean your brushes is to wipe away the excess paint, do a light wash with oil (linseed is okay, but Safflower or Poppy Seed oils are lighter and work a bit better), then wash the brushes with warm water and a mild soap. You have to try these methods to see which you prefer. Much depends on your quality of brush, whether they are natural or synthetic fibers, etc.
A somewhat altered method of the above strategies is to wipe off excess paint, then swish in paint thinner until all color is removed. Use a cleaning jar (Lion Silicoils are better since they have a rust proof metal coil in the bottom rather than a screen, which can be rough on brushes) half filled with thinner and rub the brushes across the coil until paint is removed. Then wash the brushes in a good quality conditioning brush cleaner like Master's Brush Soap. Using Master's is a good idea on all of your brushes once they are cleaned no matter the medium since the conditioner is so good for brush longevity.
One final thought on cleaning is to use Murphy's Original Oil Soap. This stuff is so good that it will usually remove dried oil paint if you soak your brush in it full strength for a couple of hours. Just make sure you rinse thoroughly.
Lastly, drying your brushes is very important. Never dry your brushes standing them vertically with the hair up. Over time this can loosen the hairs and they will fall out. It's best to dry them horizontally or leave them horizontal if you are not cleaning and then plan to paint the next day. They can be stored vertically hairs down, but do not rest them on the hairs. This quickly misshapens the brush. After cleaning and removing most of the moisture, reshape the hairs before you set them aside to dry.
Any of these methods work well to keep your oil painting brushes in top shape for a long time.