Quality over Quantity
My athletes hear me say this a lot as its something I truly believe in. I want the focus to be on the QUALITY of the training session, rather than the QUANTITY of it.
I would rather an athlete complete 5 perfect repetitions of an exercise, than 20 half-assed repetitions. Quality reps means you are getting exactly what you are intended to out of the exercise. Let's take the squat for example. By squatting through a full range of motion, you engage the appropriate muscles, activate them, and reap the benefits of the whole movement. If you start cutting the ROM short, you lose out on activating some of those key muscles.
You will always run into a bit of ego in the weight room. Some athletes are convinced they are moving big weight with perfect form, but sometimes it appears they ditch the technique for bigger numbers. This is where I come in to knock the ego down a couple pegs. I explain that in order to get the most out of the exercise and build all that awesome lean muscle mass, you have to go through the movement fully and not sacrifice the technical side of things in order to lift more weight.
When I'm coaching, I would rather focus on fewer exercises and drills; do them properly with adequate rest and recovery, than try to pack in 10 things in the same amount of time. Less is more.
This brings me to my next point - performing repetitions of an exercise under a time constraint. There is totally a time and place for timed sets, but that does NOT mean your technique can go to sh*t just so you can get a couple reps more. I often explain to my athletes that I'd rather see them get less reps maintaining their technique, than blow through a ton of messy ones. This allows the athletes to benefit from the exercise and save themselves from potential injury.
Now let's put it into a different context - would you rather eat a bowl of real, authentic homemade Italian pasta (with fresh made sauce), or eat three cans of year-old Spaghetti-O's?
... QUALITY over QUANTITY always wins.