I have a writer friend who once won a sizable grant. After that, every time she mailed a manuscript, she tried to go to the same postal clerk who had stamped her prize-winning manuscript.
In my submission-tracking spreadsheet, I use slashes to represent works that I've sent out, checkmarks to represent those that are accepted, and X's for rejections. The thing about slashes is that they can be forward slashes (/) or backslashes (\). At times, I have convinced myself that the manuscripts I track with forward slashes have a greater chance of acceptance; at other times, I've decided that manuscripts tracked with backslashes have a better chance of success. (Most sensible people know a well-written, properly targeted manuscript has the best chance of all, but that's not as easy to control as the direction of the slashes.)
Both of the above fall into the category of superstitions, but there are also writerly quirks and habits. Some of mine: my penchant for scribbling notes on receipts and candy wrappers; my need to have a glass of water on my writing desk at all times; and the way pens accumulate on my desk until the very moment I want one, when they all suddenly migrate to other parts of my office.
What are your writing quirks?