Let me take a stab at this (spent 16 years powerlifting).
First, I divide my days into a four-day week, like Rip. However, I divide it into two pairs of push-pull days. On Mondays and Thursdays I do all pushing execrises- bench, incline, shoulders, and triceps. On Tuesdays and Fridays I do lat pulls, bicep curls, reverse curls, and dumbbells.
You start with the heaviest lift and work to the lightest lift, because as you move through the routine you will grow progressively weaker. In this way, you maximize the amount of stress you can place on each muscle group.
Sets are usually 5 sets of 5 straight through, with the exception of the lat pulls (done on a lat bar for me), which is done to the tune of 6 sets of 10, 3 wide grip, 3 narrow grip.
At first glance, this workout looks imbalanced to the Monday/Thursday side, but in truth it isn't. One of the hardest lessons I had to learn about physical training is that most people do not understand the rest/recovery cycle. It takes at least two days before any muscle group can be stressed again, and three is better (and the older you get, the more this time increases). More, free weights usually require the lifter to recruit ancilliary muscle groups for balance, assistance, etc. Push exercises also tend to be the real power exercise, and exert the most stress on the body. So... if the power routines are done on Mondays and Thursdays, the workouts done the next day need to be a bit lighter in stress, and therefore a few less exercises actually helps recovery. In addition, the mental aspect of having to be "up" for the workout is only really a factor on two days, helping to keep the appetite up to push. This leaves three days for cardiovascular work (if you're so inclined), and it all fits in a nice, neat, 30-minute cell (if you are efficient).
I used to believe I had to spend 2 hours a day in the gym, little realizing all I was doing was overtraining. I dropped lifting when I finally burned out, became a runner, and dropped from 210 to 155 (skeletal on my frame). When the heel spur forced a break, I went back to lifting. In my first life as a lifter I maxed bench at 345lbs. Upon my return, I have maxed 360, this on a quarter of the time and at age 40. I have now moved off the weights again for a while for a return to running, and will probably pop back-and-forth between the two in cycles.
Rip alluded to acclimation- this occurs in all sports after approximately 6-8 weeks. What happens is your body by this point has grown accustomed to the type of work you've been doing, so significant gains drop off. It's at this point you need to plan to shift emphasis, mix up the routine, or pick a new activity altogether. I've found this to be consistant in swimming, running, biking, lifting... basically everything I've ever done. Most serious athletes plan months into the future, including periodization into the plans.
As this dissertation is already way beyond what most people will read, I will stop. If you want to know more, ask, otherwise I yield the floor.
