This isn't Rocket Surgery!...
You gotta mix it up.
I think we all agree on that. So just about the time your body is getting used to a routine, say 90 days, switch everything up, in order to confuse your muscles and CNS into having to adapt. Or alternate every other workout. Heavy. Light. Heavy. Light.
That's the only way to keep growing, IMO.
Though I do alternate between very heavy periods of training and much lighter periods for a shorter time while I am building back up to another heavy burst, I disagree that routines should be switched up often. The only time I switch out an exercise is when I can no longer progress on it (when I cannot add more weight to the bar for the same reps I did last time, or do a few more reps at the same weight as last time).
As for switching out my routine, I have stayed with the same routine for 10 months, and even though I just came off a 3 month layoff of mostly lighter lifting, including probably 4-5 weeks off altogether in that time due to having a baby...I have gained about 20 lbs of lean mass in about 6 months of effort. This was after having been stalled on gains for a year. I haven't done any cycles either.
I think most poeple "mix it up" too often. They keep changing their routines and wonder why they're the same weight or only 5 lbs heavier than they were a year ago. How do they measure progression? They usually don't.
You're right, this ISN'T rocket science!!! No matter what Flex magazine or some "bodybuilding guru" tells you. Most people make the mistake of overanalyzing and second guessing themselves, and do one routine one month and Flex magazine's routine the next month, and something else they found online the month after. How the hell do you know what's working if you keep switching it every month?
Most of these guys barely grow, if at all...or they load up on a bunch of steroids and in two years and 5 cycles later, they've managed to gain a net of 25 lbs. Now instead of 6' tall, 175 lbs, they're 6' tall, and 200 lbs! What a waste! They could've saved the gear and just eaten more and trained right and got their 25 lbs in probably 12 to 18 months.
Bodybuilding is about progression. Keep progressing at an exercise or a program until you cannot progress anymore. Then, find a new exercise (for that bodypart) and progress at it until you cannot progress anymore, then go back to the old exercise and you'll find you can blow away your old best effort.
Think about it this way, if you were able to add just 5 lbs. to your bench press every week, in a year (52 weeks), you'd be pressing 260 lbs. more than you could a year before. So if your 5 rep max was 200 lbs., your new 5 rep max would be 460! But maybe you took a week off every 8 weeks, and another week off at christmas, so you only had 46 weeks of 5 lb. progression on your bench. So, instead of a 5RM of 460, your 5RM is 430. You suck!
Wouldn't you just hate to add another 230 lbs. to your bench in a year? Inch by inch. Those 2.5 lb. plates are your best friends in the gym. That progression would probably equate to another inch to inch and a half of thickness (new muscle) on your chest!
Think I am lying? Go out and try it! Prove me wrong! Even if you didn't add 230 lbs. to your bench in a year, I bet you'd get a hell of a lot bigger doing it this way than whatever way you were doing it.
(
the "you's" in this post are not directed at anyone in particular (even though I replied to your post, halfcenturian), I'm just talking to anyone who's listening and hopefully someone learns something)
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, I will have to "dispose of". 
you're 6'4'' and 240 lbs 
me?
