Water, and snacks. More water. Don't get dehydrated, it causes you to be less mentally alert.

Confidence. Understand that while you aren't seeing deer right now, at ANY second that buck could step into view, and you are in the perfect place to do it.

If you are hunting dark to dark...you are gonna get tired. Take a break. Sleep in one day and head to the woods around 8:30 or 9:00. Don't put too much pressure on yourself.