Keep makeup brushes together and within easy reach by organizing them in a pretty little cup...
...or maybe a re-purposed glass jar or vase...
...and if you have brushes that are shorter than your container and consequently are inconvenient to grab, take a cue from the people over at the Makeup Box Shop (photo above right) and Carlee of Deliciously Organized (photo below) and fill a portion of your container with little beads or sand (or marbles). Doing so will help brushes stay upright, in place and optimally accessible. 
How do you like organizing your brushes?