I can't explain it. There's just something about sitting down to write, a favorite drink within reach. It makes the task ahead of me - hurdling through that dogged plot point, smoothing out the choppy prose or dialogue, etc. - easier to face; it helps my mind separate - somehow like a mere beverage provides a little retreat for me - and home in on my writer's voice.
My favorite is this:

Poured over ice and sipped through a straw, it has to be mocha. Which will come as no surprise to my friends and family. But it's an expensive habit. A carton of four bottles, though still cheaper than hitting the local coffee shop, is upwards of $6.00 at some of my local retailers. I'm willing to spend the money, but it adds up. And quickly.
But I recently found a different, worthy drink. It's a yummy alternative to the Starbucks (I didn't think there'd ever be one) and, blessedly, happens to be less expensive, too.

A friend introduced me to this Chai Tea. Can you see there on the front? It's a mix of black tea, vanilla, spices and honey. Anyway, she made it with half milk, half concentrate (as the directions call for) and ooh, dawg, is it tasty! I use 2% milk and serve it cold, as is my preference with teas and coffees, but my mom drinks hers with soy milk, heated. Either way, it's just over $3 per carton (at good ol' Wal-Mart, next to the teas and hot chocolate mixes), and it makes a gallon all said and done.
Mmm. I'm enjoying a cup of the latter right now...
So what's your writing beverage of choice?