Best: Tough to pin down a single one, but the notion that you should look to people who’ve done what you want to do for advice and guidance is probably it. This helps filter out a lot of distracting noise, especially for a new trainee who might be drawn to something they find appealing or to someone whose advice might not be the most applicable. In other words, look at the totality of methods that’ve produced the results you’re after before you decide the wheel must be re-invented.
Worst: “Don’t use your strength”. This applied to BJJ, not strength training, but it still stands out as the worst training advice I’ve received. The correct advise is “refine your technique”, which is true for everyone no matter how much muscle they have. “Don’t use your strength” is, in practice, upper belts expressing disappointment that their stuff didn’t work. It is so vague and potentially misleading that it should never be said to anyone of any strength level on a BJJ mat unless it can actually be explained in meaningful detail.
It is one thing to say “don’t use your strength” if a new student is having success with brute force, absent any level of technique, like an adult flinging a child about easily. It is another thing to say it if a trainee is having success with the application of techniques in part due to strength disparity.
The antidote to that was training under an instructor who insisted, INSISTED that I smash anyone with a blue belt or higher, along with most male white belts. You do not want to grow accustomed to moderating your hip strength simply because it is a cheat code with low-level grapplers. Nor should you grow accustomed to moderating your pressure simply because lower-level training partners want to call it “using your strength”. Pressure is pressure and more pressure is good technique, whether through strength, size or refined technique. It is always a combination of those for everyone, no matter how much they bench.
In very simple terms, train like you intend to fight, not how your more sensitive and less seasoned training partners would like for you to train.