mine is actually something I learned about in therapy. wet a washcloth with cold water, then put ice cubes in it. there are various places you can put it to cool down (inner part of your elbow, neck, lower back, etc.) but the first one I was taught was actually the pulse point under your chin, because it also serves a purpose of calming you down. if you're angry or anxious, it might help, on top of cooling you off.

a second thing that I've done is, before bed, putting my comforter and/or my pillowcase in the freezer. only really works depending on size of the freezer and comforter, though.