If no is expensive, yes stops carrying information. A partner who cannot comfortably refuse is not really consenting — they are managing you.
Be clear, not vague
Hinting, going quiet or hoping they lose interest is harder on everyone than a plain answer. Vagueness reads as rejection of the person rather than the moment.
Give the reason if there is one
"I am exhausted" or "my head is somewhere else" tells your partner this is not about them. Where there is no reason, that is fine too.
Offer a door, not a consolation prize
If you want to, later, say so and mean it. A rain check nobody intends to honour is worse than a clean no.
Receive it without punishment
Sulking, withdrawing or going cold teaches your partner that honesty is costly. They will start saying yes when they mean no, and you will never know.