If you re going to use a brush, try using an older one you won t miss, or a disposable one. A paper clip is great for making lines and tree branches. Just pull the clip apart and dip into the mask. Because it s metal, there s no problem with clean up. Wait til the mask is dry and pull it away from the tip.
Stems are great since they make the most natural looking stems and branches.
You ll never run out and you ll never have to buy one. A weed stem, while not as sturdy, works just as well. A calligraphy pen, with interchangeable tips is a good one to use if you are masking highlights out on hair, for example. Cleanup is just as easy as with the paper clip. Let the mask dry before removing.