Like athletes, many scholars develop rituals, charms, and neuroses related to our performance: clothes we wear when writing or researching, places we sit, mugs we drink out of. Most of my own habits feel incidental: I wear a version of the same outfit (with warm- and cold-weather variants) whenever I work at home, but that's just because it's easier not to have to think about what I'm wearing; the outfit itself doesn't have talismanic properties.
But the one thing I absolutely cannot work without are legal pads. I write only on yellow legal pads, and if forced to write on something else (I've forgotten to bring a pad to a rare book room, and they give me loose sheets of printer paper) I'm itchy and unhappy and transfer my notes as soon as I get home.
I also need a decent writing implement, and though I'm slightly more flexible about those, I've used Uniball rollerball pens (with a micro nib) in their various incarnations since I was 18 and they remain my strong preference. When I use a pencil, it has to be a a mechanical one; I can't deal with the erratic and changeable line precision and messy writing that results from a conventional pencil.
And if I had to add a third item to the list, it would be my lap desk. My brother bought it for me several years ago, and though I can work without it--there are always desks and tables--I work best in an armchair, on my sofa, or in bed. I take my lap desk with me when I travel by car. It's more of a luxury good than my legal pads, but it makes me happy.
What are your own fetish objects or rituals for writing?