This is one of the most important things a bodybuilder needs. Rest gives your nervous system a rest from the intense stimulation from training. Over training can result in a lack of rest, which also leads to a lack of intensity. To bring up your intensity, take a week off and let your nervous system a rest so your mind is fresh for your next workout.
Second, get more sleep! Yawning when you walk into the gym is a sign that you won't have a good workout. Lack of sleep can result in a lack of intensity, which results in an overall crappy workout. Try going to bed earlier or sleep within the hours where you feel refreshed in the morning. Supplements that can help the quality of sleep are ZMA and Valerian Root.
The next topic is your surrounds. How is the general mood in the gym you are in? Is it filled with muscle heads pounding away at ungodly weight or fat women gasping for air on the treadmill? Being in an environment that is subtle is not going to help get your intensity up. You should be working around guys that are striving for the same intensity you are, not around a bunch of old ladies staring at you every time you drop a five-pound plate. Try going to a gym with a hardcore attitude; but if you end up at Bally's, do the best with what you got.
Is your gym playing Slipknot or Sinatra? Music can make a huge difference in the intensity you have. Walking into a gym with classical music isn't very motivating, will most likely make you feel lazy and make you mope around. Having good music being played can help your greatly in busting out those last few reps of rock-bottom squats. If your gym doesn't play good music, just bring your own audio player. Make sure you have headphones that clip onto your ears so they won't fall of in the middle of your lift and make you look foolish. Also make sure you can have an audio player holder so you aren't walking around with holding you MP3 or CD player in your hand. CaseLogic.com sells the best audio player holders I've seen.
Thermogenics can help give you the extra energy you need to have a really great workout. Though they are mostly listed as fat burners, thermogenics also give you crazy energy. These work by increasing your heart rate, thereby increasing your metabolism and making you feel more energetic. Though caffeine is used as a pick-me-up, it is the ephedrine that really gets you in gear. The best energy booster I have tried is Ripped Fuel. Though it's not so great as a fat-burner, it really can make you a beast in the gym.
What good is an energy booster if you have no fuel to run on? Making sure you have enough complex carbohydrates in you can determine if your can go all-out for 30 minuets or an hour. About 30-50 carbs a half hour before your workout is adequate for sustained energy throughout your workout.
Creatine can also help by giving your muscle endurance. This is most useful when you are doing higher-reps (from 8-15) because that is when your energy reserves start running out of Creatine. Creatine in serum form is probably a better choice for increasing energy because of the reported "energy boost" you get after you take it. Serum has this effect because when the liquid is absorbed under the tongue it goes straight to the blood stream, while the powder must go through the digestive system first.
Though you may not consider this as a factor, I think what you wear can help give you the ego needed to go hard in the gym. Wearing clothes that shows off what you have done with your body can make your very motivated. When walking into the gym and seeing people's eyes following you, it can make you want to train like a beast so you will pick up more eyes and have a nice little collection. Also, seeing your muscles contract and relax can be very hypnotizing and will make you want to squeeze out a few more reps. Buy a few tanks and see if it helps. If you are not 'huge' yet, this can be motivation to train until you full that tank you are wearing. Remember the old story about how Arnold cut off the bottoms of his pants so his small calves would be exposed for insults? He did this for the motivation to improve his calves, and look at them now!
To conclude, these all are factors in the amount of intensity you will exhibit in the gym. Remember that bodybuilding is a sport where the only coach you have to guide you to success is yourself; so it can be easy to get knocked off that road. Take the challenge and show you have an iron-will to succeed. If the intensity and quality of your workouts have gone downhill, be a man, admit it and use these tips to improve yourself!