Once you buy a good paintbrush, you should consider it a lifetime investment and treat it accordingly. By far the most important thing you can do to keep one in good condition is clean it thoroughly after every use.
But brushes get harder and harder to clean because paints get better and better, especially latex paints. Today many paints are hybrid formulations combining watersoluble latex resins and oil-based alkyd resins. Those alkyd resins improve glass, durability and adbesion, but they won't wash completely out of a brush in water. you'll need a hybrid cleaning technique. Here's one worked out by DuPont:
Start by washing the brush in detergent and warm water. That takes care of most of the water-soluble latex resins. (Since water-based paints tend to dry quickly, paint often begins to set up on the brush while you are using it, especially at the heel. So check you brush. If you see caked paint stuck to the bristles near the ferrule, rake it away with a wire brush.)
Next, soak the brush in lacquer thinner. This contains powerful solvents that soften alkyd resins, even if they've begun to set. Work the lacquer thinner into the brush thoroughly, then rerake with the wire brush to remove any caked alkyd resins.
Rewash in detergent and warm water and stroke brush on a clean surface to remove any traces of paint that may remain. Then take the handle of the brush between your palms and rub your palms back and forth, spinning the brush to remove excess water. Hang the brush to dry, bristles down. When dry, comb the bristles with an ordinary comb. Start with the coarse teeth and finish with the fine.
What about other finishes? No new techniques are needed: Oil paints and varnishes come clean in paint thinner or turpentine. Thinner is a lot cheaper. lacquer cleans easily in lacquer thinner. Shellac comes off with alchobol or ammonia. Ammonia saves you money. Use paint thinner for oil stains. And no matter how you clean any brush, finish up with final wash in warm water and detergent.
But how about the expensive brush you forgot to clean, the one with bristles hard as rock? The cheapest answer is to suspend it in a solution of trisodium phosphate and water. TSP is sold at paint stores. Use four ounces of TSP per quart of water. Put the can on the stove and let it boil slowly. Don't let the bristles touch the bottom of the can; the heat may damage the delicate tips.
Now and then take the brush out and rake the bristles with a wire brush. Keep soaking and raking until the bristles come clean all the way up to the ferrule. Finally, wash in warm water and detergent, then spin dry.