I want my child to take me seriously when I say "No". However I suspect she will not listen to me when I say "No" if I say it all the time. That is why I have decided to reserve it for a small number of situations. The main one being when she does something to hurt someone including me, like hitting, slapping, biting or aggressively pulling hair, specifically, when she does these things in an angry or frustrated way, rather than in a playful way, and when her aim seems to be to hurt and not to play. Then I think it is best to say "No" in a stern voice and put her to sit on her own for a few seconds. The other occasion to say "No", which I hope will never even occur, is if she is in a very dangerous situation for example about to run out on a road, touch a very hot object etc and I can't get to her on time to remove her physically, again, I would hope such a situation will never occur.
All the other times, I have resolved not to say "No" and to have alternatives. That way, she will understand that my sternly voiced "No" is a very serious word to be taken seriously.
Alternatives to "No":
"careful" (in a warning, concerned tone) - this is for when she is climbing on something or has put herself in mild danger. If the only danger is of a mild bump, I say "careful" but let her continue. Sometimes she has fallen, and now she understands the meaning of "careful".
"you want x? wait until..." - this is for when she "asks" for something which I don't want to give her just then because it is not practical or convenient, but I will give her shortly after, for example "asking" to be picked up, for milk, to play with me, or to play with something which is generally safe/ok to play with but I happen to be using it at the time. Then when I have finished whatever I was doing or got to wherever we were going then I say "now I can... ". I think it is important to offer her whatever it was she asked for when I have reached the milestone I agreed to so she knows that if she does wait she does get it.
"ah ah" - this is for things which she can't touch because it is not safe or she might break it. However, I think it is best to avoid continuously saying "ah ah" by ensuring that such things are generally out of reach in the first place. Also I think it is not good to be too strict about what she can touch so as she is not discouraged from exploration.
"not for eating" (accompanied by yuk expression) - this is for when she can touch something and play with it, but not put it in her mouth, for example paper, leaves in the garden etc.
I will probably add to this list over time of course. As she doesn't understand complicated sentences I think the emphasis should be on easy to understand words like these. I sometimes find that the examples given on AP websites of what to say to children in different circumstances are often overly complex for young children. It would be nice to get others' ideas on simple vocabulary which can be used for young children.