keep sb sweet

keep sb sweet {infml} (to say or do pleasant things in order to keep somebody in a good mood so that they will agree to do something for you) — любезничать с кем-л;

Example: You'd better keep her sweet if you want her to lend you the money. (OALD)

Related vocabulary:
[kiss up to sb]
[play up to sb]
[butter sb up]

shine up to,
suck up to,
lay it on (thick) with a trowel

[ходить на задних лапках]
[wait on sb hand and foot]
[dance attendance on sb]
[walk on eggshells]
[плясать под дудку]