Good paintbrushes are worn in paintbrushes. For the first few times you use them they will shed bristles. It is not until a brush has been used a few times that it reaches its optimum state. Therefore an old paintbrush is a good paintbrush and so proper storage of a good paintbrush is not just about saving you money, its also about achieving a better finish.
Leftover paint will harden and destroy your brush,so cleaning the brush is important. Wash emulsion paint out of your brushes using warm water and washing up liquid. Ordinary kitchen sink detergent is just fine for the job.
Wash ALL the paint out of the brush. It can take 10 minutes to wash a brush properly, but do not think about skimping on this task.
Dab it dry, wrap the bristles in a strip of paper and tape it down. Now hang the brush from a nail or hook until you next need it. If you follow these instructions, a good brush will last a long long time.
Finally, lease ignore anyone who suggests you wrap your brush in a rubber band. The rubber will eventually degenerate and stick to the bristles, ruining the brush.