The other issue I've had is when trying to correct form problems. I have been accused of being to critical, even though I try to stay positive. Any suggestions on this?
Deffinitly agree with what Peter said. It's very easy to overwhelm people. Even experienced lifters. For most people focusing on more than 1-2 cues at the most will be enough.
It can help to explain form cues before you use them. If i'm teaching a newbie to squat, I explain to them what "hips back", means, and I explain what "knees out", means. I'll just say, "i'm going to explain a couple of things first, so that if I say them you'll understand". For benching I show people how to get they're shoulder blades down and back before they even start - you'll be surprised at the little things you as an experienced lifter just "know" which newbies will not.
For squat there's a couple of things - I just concentrate on hips back and knees out. I limit the depth to a safe ROM with a box/step or whatever. Really don't worry about depth right now. You're just trying to get the hips and knees doing what you want them to do.
I find even holding a light DB at the chest makes most people keep the chest up (back straight) without even having to mention it to them. If the chest still does cave, normally just telling them to look up a little will do the trick. That's another thing - watch the eyes! Some people seem really unstable and you can't work out why, then you spot that they're scanning the room repeatedly throughout the set! It's amazing the difference it makes when get someone to fix they're eyes on one spot, and focus.
I'm possibly contradicting myself by overwhelming you here!
Also, when someone isn't "getting" something, as Peter mentioned, just act like it's your fault. I believe this anyway - if someone isn't getting it, I believe i'm not coaching them properly. Of course some people are harder to coach than others but, that's the fun of it, really.