10 great tips for Big Wall beginners. I would stress #2.
2 - Not bringing enough water
Being on a big wall is a lot like being out at sea. If you didn't bring enough fresh water, you're in trouble. The unfortunate thing about it is that water is just about the heaviest item you bring on a wall, as well as the most important. The advice given to me before my first big wall was to bring enough water for at least two days more than you're planning on being up there. This gives you some wiggle room in case you drop something, a bottle breaks, or you are way slower than you thought—all very real possibilities.
In my experience this should be shouted at every new Big Wall climber. It's a tricky balance to bring enough water and not have your haul bag get too heavy...but if you're reading this...err on the heavy side with more water.
My second tip if you want to shave a few pounds in water...bring Jolley Ranchers. When you're dehydrated and wishing you could drink the Merced river, sucking on these tricks your body into thinking you are a little more hydrated than it is.