Mix it up, including negatives and various cardio workouts.
For example, focus on your back and chest during one workout, but the next time you want to hit those muscle groups, try chest, shoulders and biceps, then a couple of days later go for back, triceps, legs and abs.
The following week, do back and chest one day, legs, shoulders, biceps and triceps a couple of days later, etc.
Then, take a couple of days out of a workout week to focus on all of the muscle groups but lower the weight and work on negative lifting. Oh, and regardless of your goals, you should start every workout with a little cardio. It's important for heart health, but it also helps to loosen you up and oxygenate the blood prior to lifting.
Note: The ACE Personal Trainer bible claims that when you exercise after eating, you're blood flow is in competition between providing oxygenated blood to your digestive system and the exercising muscles. Don't eat a meal 1-2 hours before working out b/c oxygenated blood flow to exercising muscles is essential.
For energy, I recommend ingesting a banana and a low sodium V-8 (an acquired taste, I know) 20-30 minutes prior to working out. Protein drinks are great, too, but they get to be expensive and can cause GI problems if not consumed in moderation.
Workouts like this help to maintain a balance without exhausting yourself.
Have fun, don't go for overkill, eat well, and get the proper amount of rest.