I just write wherever I happen to be. After reading several professional authors write about how it's not a good idea to construct limitations around how and where you can write ("Must be at my favorite coffee shop with a large cafe latte, a yellow legal pad, and three sharpened #2 pencils") because someday you may need to write under other conditions, I've made a deliberate effort to make my writing as condition-free as possible. I do, however, do almost all my writing in Evernote - I'm considering the idea of doing some writing on paper just to keep things different.