First and foremost, try to clean and care for your brushes after each use......you know DO AS I SAY AND NOT AS I DO! LOL I keep a bar of Scrubby Soap beside each sink in my workshop, my store, my classroom and my house. So really there's no excuse for paint to dry in my brushes. Scrubby Soap is not only great for cleaning your brushes after each use, but also for cleaning your hands, spot cleaning clothes (gets paint off!), and so much more. Others have suggested that Dawn Dish Detergent is also great for cleaning brushes.
So, if you follow me, you know that I use mainly water based paint/stain products. If you allow water based products to dry in your brushes, here's what you do. I immediately soak them for at least 8-12 hours in Murphy's Oil Soap and warm water. Then I take a paint brush comb or a flea comb and finish removing the rest of the paint residue from the bristles. Sometimes I also use a Scotch Brite Scrubby Sponge as well.
If I am reviving brushes with oil based products in them, I use mineral spirits for soaking then the paint brush comb or a flea comb and finish removing the rest of the paint residue from the bristles. Always use mineral spirits in a well ventilated area, even though this is low odor.