One bit of advice that writers hear again and again is: write what you know.
I have never found this to be helpful. Some days I wander around not even sure of what I know (am I alone in this?). Anyway, I've always found it to be more helpful, and more interesting, to write about what you don't know, but are intrigued by. That way you are curious and enthusiastic, which will automatically make your writing more alive when the words start hitting the page.
So, think of something you don't know much about, but wish you did and then start doing research. Do a Google search, go to the library, talk to experts, read books, watch a documentary. Maybe a character in your latest short story loves Japanese food, but you've never tasted it yourself. Great! Read cookbooks, visit a Japanese restaurant (or two) and then learn how to make sushi (and invite me around for dinner). Learning about something new can be a great way to wake up yourself and your writing!