One thing I've noticed during some of the painting lessons I've conducted was the poor condition of many painter's brushes. Painting is hard on brushes, which is not suprising considering that the working end of it is just a bunch of hairs held together with glue and a metal crimp. After only a few models, those hairs are subjected to more abuse than Steve Carrell's chest in "40 Year Old Virgin".
Since I always advocate buying the best brushes you can get your hands on (either the Windsor and Newton Series 7 brushes, or the Raphael 8404 Kolinsky Sable Brushes, but I've also had some success with the new GW Kolinsky sable brushes too), you're paying a premium for premium results, so you want your brushes to last. Here are a few tricks and tools that are pretty much common knowledge for many painters, but not all:
1) Try not to let the paint go past the metal ferrule. You should work with the first half to perhaps 2/3rds of the way up the bristles. If your paint reaches the metal ferrule (the metal shaft crimping the hairs together, it can work it's way into the base of the hairs. Once the paint dries in there, it will push the hairs apart and your brush will start to splay out. Now, don't panic if wet paint gets in there... just remember to rinse your brush thoroughly before the paint dries in there.
2) Don't use hot water to rinse your brushes. The hairs are held together with glue and sealed with the metal ferrule. Heat will soften the glue, causing the hairs to get misaligned. Room temperature water should be good enough, unless it's an old drybrushing brush that doesn't need to hold a decent point.
3) When drying your brush on paper, drag it near-parallel to the paper, while giving it a slight twirl at the same time. You don't want to mash the end of your brush on the paper, splaying the hairs all over the place. By gently dragging it almost parallel to the surface of the paper, the paper will draw moisture from the brush, without damaging the bristles. A slight turning of the brush while doing this will cause the hairs to form a nice point at the same time. Also, see if the brush is leaving any colour on the paper. If so, it'll probably need another quick rinse in clean water. Any pigment still in the bristles usually means there's still a trace of paint in there, which can dry and damage the hairs.
4) Use brush soap at the end of a painting session to really clean the hairs. Just like the hair on your head, a gentle soap will ensure any oils, greases, and dirt will be released from the hairs of your brush, making sure they're nice and clean for your next painting session. I like to use Masters Brush Cleaner or Mona Lisa Pink Soap. Both work amazingly well, although I'm finding the pink soap marginally easier to use than the solid block of soap in the Masters container, since you don't have to wet it down first.
After every other painting session or so, or if your next painting session will be a very long time from now, I recommend you give your brushes a good rinse (under room temperature water, of course) in the sink, gently massaging the hairs with your fingertips. Before you lay them out to dry, work a tiny bit of hair conditioner into them. Yes, I said hair conditioner... the same stuff you (or your significant other) uses in the shower. What's good for your luxurious mane should be good for the tail hairs of the Siberian Mink (the poor bastard lost a chunk of his rearmost end so that you could paint miniature masterpieces). Form the brush (with the conditioner) into a nice point, and it'll hold it's shape when dry, as well as strengthen the hairs and keep them supple for your next paint session. Remember to give it a good rinse before using it again. I'm partial to Pantene Pro, but that's just because I really like to spoil whatever hairs have been loyal enough to my head to stick around.
5) Trim away any hairs that refuse to form a point. Most of the time, these traitorous non-conforming bastards will weaken at the base and break off on their own, but if they don't, take some pointy scissors and just cut them away from your brush. If you don't, these f*ckers will make a mess of whatever part of the model you're trying to paint. Like the hairs that grow too long for your ears and nose, I advise you kill them with extreme prejudice.
Anyway, I hope that helped. In the end though, no brush lasts forever, no matter how much you love and nurture them. Best case scenario, they get skinnier and skinnier as they lose a few of the weaker hairs, while maintaining a nice sharp point. Typically though, it just gets harder and harder to maintain a nice point, and then they're good for nothing but drybrushing, mixing paints, and perhaps as chopsticks for really tiny hands and even tinier appetites.
When this happens, go out and treat yourself to a new brush to replace it. You deserve it, and nothing is more frustrating (and time consuming) than struggling to do a decent paintjob with a crappy brush. Our painting time is limited and valuable, and our models are not cheap either, so it pays to use the best tools we can get our hands on. It'll save time and money in the long run, and you'll be happier with the end results.