Have a brush that's  stiff and full of dried up paint?

Dip your brush into a gel paint stripper. Let it sit for about 10 - 15 minutes. 
With a paper towel rub the gel into the bristles, and then 
Run it under water. Repeat if necessary.

If you only have the liquid kind of stripper, pour some in a glass 
and let your brush sit in it. Use the same directions as above. 

Need to get wet acrylic paint out of your brush?

Wipe excess paint on a piece of paper or paper towel, and run your brush under water. 

  This is ONLY for acrylic and other water based paints

Wipe excess paint on a piece of paper or paper towel. 
Moisten a paper towel with paint thinner, and roll your brush in the moistened area until it is clean.