I am a trained classical artist and I still have some of the original brushes I bought in 1985 when I went off to school. Always remember high quality brushes are best. Red sable being one of them, however, I will say some of the new type of synthetics have gotten much better and do just as good a job. There are several things you can do:
1). Never let your brushes dry out! By this I do not mean just the obvious but, if you paint with the same brush for extended periods of time, clean it periodically. The capillary action of the paint will carry up the bristols to the base, over time this will build up and cause the hairs spread apart.
2).When cleaning the brush I always clean them in the palm of my hand gently, never scrub them on the side of the solvent jar or the bottom. Don't force the brush in an unnatural manner to the bristols always only parallel to the bristols. If a brush needs to soak don't stand it on the bristols, use a container big enough to accomodate the brush, or one with a lid so the brush can lie flat.
3).Clean your brushes thouroughly with the appropriate solvents for the appropriate type of paint. Acryilic to acrylic based solvent, oil to an oil based solvent. Have seperate sets brushes for use with acrylics and another set for oils. Over time the solvents will soak in to the fibres of the brush and will leave a residue that will contaminate the other.
4). After cleaning with the solvent, clean them again with dish soap and water, it will help with the residue left behind by the solvent.
5). Reshape the brush especially the fine tip brushes, you can do this gently with your fingers, a soft cloth, or with the super fine, I will draw them through my lips
Fine tip brushes are the most difficult to keep, always treat the gently, don't use them to mix paint only use them for painting. Reshape the point after each use and protect them with a plastic sheth.