Listen to your body. If it hurts or just isn't putting out, you probably need a good rest period. Other things that help me is changing the routine around. I do this once a month. This month I am trying double split workouts. I may hit the gym at 4:30 for an hour of Hamstrings then go back about 8:30 and hit my back. I used to do these together in one workout but I have wanted to hit my Hams hard and then I am too spent for back, bi's or anything else. Go home have dinner, rest a while and I have a second wind.
You just have to listen to your body and mix it up. Most people will tell you, if they take a week off, they come back stronger.