When you know you will need to drink a lot to blend in, take a small hose, bought at an aquarium store, or at a medical supply store. Tape it to the inside of your arm so one end is at your wrist, and the other into a small bag, this only works with long sleeves, for short sleeved outfits have it come out at the shoulder, so you can pretend to wipe your face on your shoulder as you spit into the hose. You can get collapsible bags, strap them down somewhere easily hidden, wear baggy clothes to help conceal the bulge, and empty them in the toilet. If it's dark, or chaotic, you can have it run all the way to the floor.