Obviously being comfortable, having enough food, and having confidence in your stand location are all very important. But to help with the actual time element of sitting all day I mentally break it up into 3 four hour hunts. Morning, afternoon and evening. For instance, at 10:00AM I'll say to myself "only one more hour until the afternoon hunt begins" ect. Psychologically, it sort of tricks you into thinking it isn't as much time as it actually is. Seeing deer seems to be the single best element to keep you sitting all day though.