I think all of you are right about phrasing things in a positive way, however, the result might still be that the child does it in which case, especially when danger is involved (ie-the knife or the road) age appropriate punitive measures must be employed.
Different ones work for different kids, sometimes trial and error is the only way to find out what works for your child. Also, the severity of the act (in terms of how dangerous it is) applies here as well.
1) time out (one minute per year of age) works/ed for 2 of my 4 kids. (My youngest is too young to respond to it and my second just doesn't care if she is in time out. My eldest, who time out worked for when she was younger, she is now 7, puts her self in time out now when she knows she is on the verge of getting into a situation which is not good; in adult language we would call this alone time).
2)If the situation is truely dangerous (ie- my 3 year old son crossed the street on his own) immediate action must be taken and therefore time out won't work as it probably can't be done right then. I never have hit or spanked my kids, however, when he did this I did give him a swat on the tush. I am sure it did not hurt, however he cried and after he was done crying I asked him what mistake he made and he told me he is not allowed to cross the street alone. I asked him why this is a rule and he said that cars can't see him, so he could get hurt (this is the why we had told him before, it was nice to know he was listening).
3)With my 4 year old who does not respond to time out we have used either removing her from the room (sort of like time out but she can come back when she is ready to tell us her mistake) or having her stand in front of us and discussing with her (usually in a long drawn out manner in order for it to feel not nice instead of like attention) what the mistake was and why it was not a good idea.
4) with all my kids we only use the above as a last resort, prior to the unwanted behavior our house it really run on sticker charts. The kids work on one or two behaviors at a time and get stickers on their chart when they listen the first time, or play with more kids at school (not just their best friend), or learning to go pishy in the potty, or going to bed nicely, etc. HOWEVER, this does not work with dangerous behavior.
I would like to repeat that these are done after we have made the positive statement about the behavior we would like to see and the behavior is done anyway.