Totally agree the embarrassment factor gets worse. My three year old shouted out to me across the post office while I was queing (he had been "playing" next to the barrier dividing the queues a few rows behind me. I was moving up the queue but he refused to move with me and i didn't think he was doing anything that would be a problem so i let him be...) "sorry mum, i'm really sorry"
what for i asked back trying not to be too loud, conscious that the whole post office could hear our conversation, "for broking this" he said pointing to the barrier where he'd managed to detach the canvas barrier from it's pole.
Don't think it was actually broken just detached but I was mortified and amused by it all -he absolutely refused to come and stand next to me and i was desperate to get something in the post and didn't want to lose my place plus I had a taxi waiting so i couldn't do anything except hiss at him to no avail! Eventually when he did come over I felt totally hassled as he tried to "help" me write out the parcel forms -i just wanted to scream at him! But it was poor planning on my part taking him there in the first place at that particular time of day (lunchtime -busy -he was tired).
Re the regular use of "no", particularly with the younger one, I find now that she either repeats it back which makes me laugh or she seems to get riled by it so I try to use alternatives as I move her away or distract her eg "that's not very friendly/polite" that's not how we behave with our friends/make friends, we don't snatch/smack etc etc or if i'm trying to be really positive, "lets be friendly and share our toys rather than grabbing at everthing!"
maybe that sounds a bit wishy washy but even i get tired of hearing myself constantly bleating out "no"!
their receptive language is very good even if they don't have much vocab so i think it's worth being more expressive with them as they get older and can communicate more. i try to save the really firm "no", to stop them in their tracks from something that might be dangerous or if they're overly aggressive with other kids. otherwise after a while they just seem to zone it out!
i think it helped with the older one, as he became more articulate, he would come up to me and say so and so wasn't being very friendly or very polite and knew exactly what it meant from an early age.