1. Train Hard, Recover Harder
I've said it time and time again: The more you train without exceeding your capacity to recover, the more you'll grow and the stronger you'll get.
I'll go one step further and say that most people don't train hard enough to progress past the beginning of the intermediate stage. When they first start, they gain because any training represents a drastic increase compared to the hole they were wearing through the couch. But as soon as they get past the beginner stage, gains become exceedingly rare because now that their body is used to physical stress, it takes a lot more of it to force adaptation.
One of the reasons why these people fail to train hard enough to stimulate gains is out fear of overtraining (which is often just a justification for laziness).
Well, let me tell you this: True overtraining is exceptionally rare. In all my life as an athlete and coach, I've only seen two real cases of overtraining, and in both the guys were Olympians training over 30 hours per week under tremendous psychological stress.
In reality, most elite athletes train over 20 hours per week, with some even hitting the 40-hour mark. Not all of this is strength training; speed and agility work, conditioning, and skill practices are also on the menu.
Before you throw the doping argument in my face, I've seen a ton of young athletes who were obviously not on drugs follow that type of schedule. I've worked as the head strength coach of a sports academy where kids ranging from 12 to 18 would go to school from 8:30 am to 12:00 pm, then train or practice from 1:00 to 5:00 pm every day. Their programs included daily strength work, agility training, and practices cumulating over 20 hours per week. None of them were overtraining; all of them progressed quite well.
Similarly, most high-level Olympic lifters train for three hours per day spread over two or three daily sessions. Heck, Canadian National team member Marilou Dozois-Prévost engaged in two sessions daily, each lasting two hours, and would often extend these to do additional jumping or gymnastic work... when she was 14!
The benefits of youth? Maybe.
But how do you explain the case of Marcel Perron, who at 68, would lift for two hours in the morning, sprint for 30 minutes before lunch, and train for two more hours in the evening? His partner, Emery Chevrier, who power cleaned 285 and power snatched 225 pounds at a bodyweight of 170 when he was 70, would do the same minus the sprints.
And on the practical side, I've known quite a few farmers who chugged along for eight hours straight day after day, doing work that'd bury the most hardcore gym enthusiasts, without overtraining.
The problem is that most people lack the recovery capacity and don't take the necessary means to recover properly.
The Barbarian Brothers, two of the hardest training bodybuilders mankind has ever known, said that there was no such thing as overtraining, only undereating.
While not 100% accurate, they have the gist of it. Most people who think they're overtraining are simply under-recovering. While you can't make your body invincible to overtraining by pigging out, undereating, and especially undernourishment, can drastically reduce your capacity to recover.
Here are some things you can do to increase your recovery capacity: