Take a break, get some fresh air, get an outside opinion, take a nap, freewrite (jot down thoughts and ideas randomly, even incomplete/"stupid" ones, on a separate page... let the words flow freely and see if anything good comes out.), etc., etc., etc..
There's one trick I use that I didn't find in a book about writing: read the dictionary. Pick a page or pages essentially at random and look around until a word or definition catches your interest. Then, figure out correct usage of the word and build around it for a little while; see where that takes you. ...similar to freewriting, but a little more directed and, in my experience, more useful when you're already deeply pondering a particular topic/event. It works equally well with a Thesaurus (particularly the thesaurus feature in MS Word).