Hi there,
My dad, who wad a commercial artist, told me how to "train" brushes, which was something he said was a constant thing. He had brushes he used for many years.
Now the first thing you need to do is get the brush as clean as possible.
So if it is soiled with acrylic paint, you can get this stuff called "Goof-Off" from a paint store. It is a commercial acrylic and latex remover. Soak your bush in that for a few minutes and then wipe with a cloth. Then once you have removed all paint residue, then give the brush a good wash with soap and water. Rinse well and dry.
Now if you have oil paint, then do the same thing with lacquer thinner instead of Goof-Off.
Now that your brush is clean, it will probably look like it has had a bad hair day and be all frizzy. This is okay and normal. It is now time to train the brush, and this is how to do it.
Get some Vaseline Petroleum Jelly and wipe a big gob into the rissoles. Wipe out the exes on a cloth and shape the brush to the desired shape. The Vaseline will hold the bristles to shape, but if it is stubborn and will not hold a shape, then take a piece of tin foil and wrap it around the head of the brush to corral those unruly bristles.
Leave it alone for a good while , say 2 weeks to 2 months. During that time the bristles will take on there new shape. when you want to use the brush again, rinse it out in paint thinner to dissolve the Vaseline, and away you go with your fixed brush, good as new.
TrainClown