I have a favorite pen that I like to chew on when I write and edit. It's not an expensive or even a nice pen--it's just a generic soft plastic Bic pen that I replace once it gets chewed beyond recognition.
My wife always writes with a glass of ice water next sitting next to her. The strange thing is, she never drinks it. It just sits next to her and keeps her company.
A friend has a special rocking chair that she writes in because she writes better material when she moves.
My father always keeps an enormous bag of dark chocolate bits next to him when he writes letters to family and friends. Every time he completes a few sentences he pops a handful into his mouth.
Remember, it's all about setting the conditions that help make us effective and happy writers. Even if those conditions might be a bit nutty or weird.
What are your writing idiosyncrasies?