Right, so, for this I'll be showing you the wonders of brush soap. Yes, they make soap for cleaning your brushes, and this kind is $5 give or take.
Well, what does it do? It cleans, it conditions, it cooks you dinner! So that last one isn't true, but the first two are. Chances are you've got a brush that's got a tint to it that can ruin some of your colors when mixing (notably whites), or you've got paint near the ferrule (that little metal bit that holds the bristles together). Something like this prepared example that has been coated in some reddish-black paint and left to dry over the course of dinner (all of my brushes have been cleaned so I don't have a very abused brush to use as an example):
Behold what is going to fix this problem! It's fun to be able to remember what each color "ring" was for (if you wonder, green = Biel-Tan, blue = cloth on Wraithguard, red for Astorath and the blue/grey for my High Elves).
Pretty gnarly. Regardless, take about 2-3 brushfuls of water and start swirling around in the soap to work up a lather.
Good enough! Don't be too abusive when doing that as you can still throw the bristles out of whack. Now, using similar swirling motions, substitute your hand for the brush soap. The point here is to keep working the soap into the bristles. You don't have to wear a glove for this like I do, I just happened to be painting when I decided to take the pictures. I wipe excess paint off on my hand holding whatever it is I am painting and the glove makes it an easy cleanup.
Now clean the brush in your paint water like you would to normally clean paint off of it. Ta-da!
I've had old brushes with wacky bristles become usable once more after cleaning with brush soap. They might take 2-3 cleanings, but if you can save the brush? Worth the few seconds spent.
There we go, tutorial #1 of... well, many more to come, let's hope!