I am around enough people via school and various gyms to see a lot of dumb shit happen in the weight room. If I can open a few eyes, then I'll be happy.
1) Your clients aren't you. This one is pretty simple, but is often overlooked. It doesn't matter what style of training you adhere to for yourself; it doesn't mean your clients need to too. You may think that band-resisted low bar squats with a 7-second eccentric or heavy snatch combo's are the best way to get strong and powerful, but chances are that you probably shouldn't be approaching your clients training that way. Take the time to fully assess your clients and figure out what their goals are and the best way to achieve them. Even if their goal is to get big and strong, they probably haven't achieved the same minimum competencies that you have to need such specialized training practices. Stick to the tried and true basics.
2) Demand perfection from your clients. Again, it seems simple right? If you prescribe an exercise, expect that it be done with proper form every time. This is a responsibility for both you and your client; you need to coach it properly and they need to learn it. If you see your client doing something that you don't like you need to cue them out of it, or stop the exercise and fix what is wrong. If you are unable to cue them or fix it, then you shouldn't be coaching that particular lift anyway. Find a replacement lift or find someone to coach it properly.
3) Know what you're doing and why. All too often I see trainers and coach's who will start using an exercise without truly understanding it. If you don't really understand what a particular exercise or variation is supposed to be doing, then how you can successfully integrate it into your programming or even coach it the right way? If you see another trainer or coach doing something that piques your interest then stop and ask them about it. I personally learned a ton from Dan Sanzo of Northeastern a year ago just by asking him why he liked dead bugs so damn much. An exercise that I previously didn't understand at all had a whole new life for me.
4) Create a change. It doesn't matter what the change is if your client is getting better somehow. Unfortunately in the fitness field we don't just change bodies, we have to change minds too. In this regard, any little bit helps. Decrease pain, increase energy, get them to change eating habits, get them to show up on time or even just help them feel better about themselves. These things may not all have a direct impact on their goals, but it will help their over-all well being. You may have the best fat-loss program in the world, but if you can't get your client to show up on time then it doesn't matter.
Hopefully you will be seeing a lot more content come out from me in the next few weeks, so keep your eyes open. Have a great day and go lift some heavy shit!