You can only improve your fitness level by continuously pushing yourself further. Your muscles will adapt to an imposed demand, and it is only by pushing yourself past these adaptations that you will see improvement. If you can do one more rep you should be doing that rep, if you can do 5 more, then up that weight (remember if your consistent with your workouts you can increase your weight by 2.5% every 3 weeks). But at the same time don’t push yourself too hard, because that can be just as bad or worse than slacking off. Symptoms of overtraining include:

• Feelings of being washed–out, tired, drained or a lack of energy.
• Soreness, aches and pains.
• Insomnia, headaches, moodiness and irritability.
• Decreased appetite and loss of enthusiasm.

I think this is important because many people associate these feelings with a high intensity workout regime, but in fact this is not normal, nor healthy. Sure if you are in a hardcore training zone preparing for competition or a marathon you will be training at this intensity, but it is short lived, as is not beneficial to maintain this level. Your workouts should do the opposite of all the things listed. No one should be in pain, EVER! A little soreness the next day is fine (a pleasant reminder), but you should still be able to walk up and down stairs and move freely without any pain.