One phrase I've found very helpful with my kids is "they might say no." As in:

• We're at the park, Anna wants to play with one of the park toys, and another kid is already playing with it. If she comes up to me I'll say "why don't you ask if you can have a turn, though they might say no".

• We're hosting a meetup at our house, and Lily asks me if she can show the play kitchen to one of the attendees. I'll say "you can ask them, but they might say no".

When you make a request it could be anywhere on a continuum from a formality ("could you pass the salt?", "may I use your bathroom?") to a real question ("would you like to stay for dinner?", "can I have your pickle?"). The social rules around requests are complicated and variable, and even adults often have trouble with them.

While my kids don't understand the full nuances of this yet, "they might say no" indicates we're towards the "real question" end of the spectrum. It sets their expectations, communicates that whether the other person accepts the request is the other person's choice to make, and puts them in a mindset where they're prepared to hear a "no".

Sometimes, though, I also have a second audience in mind. If I think my kids are being pretty forward in asking, enough that someone might say "yes" out of obligation even if they don't want to, I'll say "but they might say no" loudly enough that the person who's about to receive a request from my kid can hear it. That way everyone involved can tell this is the kind of request that's ok to decline.

Over time my kids will figure out how this all works, but I think some experience asking for things in situations with the expectations clearly laid out is good practice.

(I touched on this some in my post on sharing culture.)