A: more sing
B: [sings]
A: more
B: I'm going to do one more song, and then night-night, ok?
A: [sad]
B: One more?
A: yes
B: [sings]
A: more
B: We did one more song, and now it's time to go night-night.

She still asks for more at the end or protests some, but because she had the heads up she's not really expecting me to say yes, and doesn't get that upset when I tell her no.

We started doing this at maybe a year? When she could just say a few words but mostly just cried to indicate that things weren't to her satisfaction. At that point it looked more like:

B: [starts to put A down in the crib]
A: [cries]
B: [picks A back up]
A: [stops crying]
B: Would you like one more?
A: [cries a little]
B: [sings]
B: Time for night night
B: [puts A down again]
A: [cries a little]

Even though she wasn't at talking age yet, she could still learn the pattern around "one more". It communicated that we were nearly done, that we weren't going to have more of the thing if she cried a lot, and that it was time to adjust.

I've been pretty happy with how this has worked out, but n=1 so this may not generalize.