Dr. Bronner's Baby Mild Liquid Soap: $2.99 for 2 floz.
I purchased this little guy at my local piercing shop, but you can find it online as well as Target and Whole Foods. It does come in other scents, but for things that are going on your skin, I always opt for the sensitive/mild version. This bottle is very small, but you really need the tiniest amount. It's not diluted, and even the smallest bit creates a great lather. There's no residue to be left behind, which is definitely a plus. Here's a quick rundown of how I use this.
Step 1: Use lukewarm water and wet the bristles of the brush. It's not the end of the world if you get some on the ferrule (metal part) of the brush, but try not to. Getting too much water inside will eventually loosen the glue, and will turn your brush into a shedding animal.
Step 2: Squirt a bit of the soap into the palm of your hand, and start massaging the bristles into it. I like to do circular motions, working it into a lather. You'll notice right away that the soap will turn into whatever color you're washing off of the brush, and that's good!
Step 3: Rinse away the soap from the bristles until the water runs clean. For foundation brushes, I repeat the cleansing process, especially if it's a more long wearing formula. I like to squeeze out the excess water and soap, it won't hurt. Rest your brushes on a horizontal surface to dry (
Ease of use is important. If it's a pain in the ass then it's a chore that will be avoided, and you really don't want to avoid cleaning your brushes. No matter what product or method you pick, it should really be something that isn't a hassle. Especially if you're the type of person that has double digits worth of brushes to clean, don't make it more of a process than it needs to be.