Ask small, specific and bounded

A good ask names the thing, the size and the deadline: "could you look at these two paragraphs before Thursday" rather than "could you help me with my dissertation". The second sounds like an open commitment, so a busy person says no in order to protect themselves from an unknown.

Make it easy to decline, too. "No problem at all if you are busy" removes the social cost of refusing — which counterintuitively makes yes more likely, because the person is no longer weighing whether saying no damages anything between you.