Sometimes brushes just get stiff and unwieldy. Other times it's due to dried paint or other things dried in the bristles. Usually this will be stuff missed during cleaning at the base where the bristles attach to the handle, but sometimes because you may have forgotten to clean your brush.
Don't fret, because all is not lost! Take a jaunt down to your local art store and look for some brush cleaner. You might also find it at hobby or surplus stores. I haven't found much difference in brands and there's plenty to choose from - you'll probably be limited to what's available. You could even try dish soaps and plastic friendly model strippers, but it won't work as well as real stuff.
You're also going to need a plastic surface, preferably with a rim. An old paint lid works great.
- Repeat all the steps above as necessary.
- If you notice one working better than the others, stick with it a few times. You'll probably find the brush will quickly give off colour, but will take some repetition to get rid of all the stiffness.
- Thoroughly clean the brush with water and wipe on a paper towel.
- You don't want to leave anything behind to continue eating away at the brush.
They won't be good as new, but they'll certainly be usable.
This can slowly eat at the glue holding the bristles in - sometimes you can only clean a brush so many times.