Beer, sometimes whisky, though that quickly starts to degrade my performance. My best run as Spy in TF2, however, came after polishing off a bottle of wine.
I've lost enough keyboards to know not to trust myself with drinks near my flailing FPS hands. Especially as my ventilation friendly PC is ready to catch anything directly off my coaster.
Otherwise, I perpetuate the stereotypes and consume gallons of tea. What What.