I recently went to Walmart to purchase a multitude of miniature composition books (they're approx 3 x 5 in.). The purpose: put one in each of my purses that I carry on a regular basis. Now, when a plot bunny happens, I can write it down before it gets lost in the abyss of my head.
I also keep a journal by my bed to elaborate on any of those ideas I had recorded during the day. The one in the picture above is the same one I have...I heart it.