One mistake trekkers make on a Gangotri with Gaumukh package is forgetting to drink water because it’s cold. Dehydration actually speeds up altitude sickness. Carry a thermos with warm water if possible. Sip regularly throughout the 18km trek. Avoid caffeine and alcohol as they dehydrate you further. The goal is to keep your energy levels steady so you can fully enjoy the moment you finally stand in the presence of the mighty glacier.