Don't sacrifice form for the number

Now that I vented a little bit last night, on to something a little more helpful.
Don't sacrifice form to achieve a certain number....what does that mean?
It seems the thought out there is that if you do threes sets of 12 that should get you the results desired. Well, my keen eye and knowledge as a trainer is looking for more than just 12 reps.
I was working with a client last week and had this individual doing push-ups. "Let's aim for 12" I said, after all it's good to have a goal and this exercise is not new to this client. So, the client starts, gets a few reps in and body posture starts to change. I could tell that chest and tricep were starting to fatigue, but I knew they could do more. This clients body started to twist a bit and they continued to push through. Awesome, love the determination, but "Don't sacrifice form for the number."
As a trainer what can I do to help this client take away the most benefit from this exercise?
I stopped the exercise and explained, and this can be applied to any exercise:
As you fatigue your body/muscles want to do what ever they can to complete the movement. Other muscles will start to try and take over or help out as a result of important muscle groups fatiguing. In the case of the pushups, transverse abs fatigued and gluteus muscles turned off. That's where I speak up and encourage the client to focus on those muscle groups, don't let them turn off, keep them contracted, make them work in order to take away the most benefit. Sure, you could let them do their thing and continue to push through, but are you then taking the greatest benefit away from the exercise? NO. Will you see the greatest stength gains and improvement in your body. NO!
So the next time you are performing a movement and it is getting hard and you want to let your body twist and contort in order to get those last few reps in, don't let it! Focus on the appropriate muscle groups, keep them firing and push through, then if you can't get that last rep, congratulations, you've fatigued the appropriate muscles and the next time you perform that movement the body will remember the pattern and be that much stronger!