After reading Carrie's post, I went straight to eBay and bought a roll of make-up brush guard myself. For those of you who don't know, brush guards help protect your brushes and keep them in shape. I normally use them after I've washed my brushes, so the bristles dry into a neat, condensed shape. This roll of brush guard allows you to cut off the exact amount you need for each individual brush.
I paid 99p, including delivery, for this roll, which is a bargain. The eBay seller I bought mine from is no longer selling them, but if you type 'make up brush guard' in to eBay, similar rolls will pop up.