How did you approach them out of interest? I see many guys.. and girls in the gym (who have been going solidly for a few months now) with shoddy form on things like bench press (wrists bent back) and deadlifts/rows (hunchback of notre dame) but never feel its my place to approach and say 'hey... sorry... can i offer you a tip?'
I was waiting for the oly bench as I needed to stand on it and shrug so I'd been watching him for 2/3 minutes. Resting for a long period and just generally seemed to be struggling. I asked how many sets he had left and then offered him a spot. Watched him closely for one set and then said:
'Have you tried it like this?' and explained pinching shoulder blades together.
If he seemed interested (which he did) then I'd go further and explain the benefits of it and coach him through it for a set.
If he'd have said 'Nah, this works well for me'/'Nope'/'Just want to do my sets leave me alone you willy'. Then I'd of just spotted him and not said anything else.
Just read the other persons reaction. But don't try and explain something if you're not confident in your capability to explain it simply and efficiently. I am PT qualified so I have spent time working on coaching techniques however approachability and your manner and tone of voice/facial expression will effect a persons responsiveness and willingness to learn. I'm not saying you need to be PT qualified to offer advice! Just if you're going to leave the person confused and no better off it might be worth waiting until you can explain things clearly.
Talking to them in the manner where you're being condescending like 'Oh dear, I can't believe you bench like that, do it like this you idiot' is a waste of time. Being polite and seeming curious as to why they don't do the useful coaching points you know is more productive.
The most important aspect is probably telling somebody WHY you're telling them to change technique. In this case it was to recruit more of his chest, remove some load from the front delts and make it more effective on the whole. 2 reps in and mid set he was already commenting about how much better it felt.
As for the lady with the kickbacks I'd seen an instructor demo'ing some closegrip push up on a bench and a kick back quite poorly. So when leaving I just asked if she was after some tricep exercises and then explained that dips would be a better choice on the assisted machine (due to more load and the benefit of a compound) and corrected her form on the kick backs.
he took it in the right way though and it was fine.
