One of the most important rules for multi-day hikes is to bring a filtration system of some sort. They come in a number of different shapes and sizes, but one very popular form is the portable straw.
While filter straws have certainly come down in price over the years, it's still nice when you can find them on sale. Right now, you can grab the Etekcity Water Filter Straw for only $15 -- $4 off its usual price -- on Amazon. This filter straw comes with a couple different accessories, including a water pouch for collecting water, a straw, and syringe for cleaning your filter.
Don't get lost in the woods without one.