Set the bar where it stretches people, not where it breaks them. Then help them reach it.

A target set too low teaches people they are not expected to improve, and one set out of reach teaches them that trying is pointless. The useful zone sits just past what somebody can currently do, close enough that effort visibly closes the gap. Finding that line means knowing the individual, which is most of the work. The other half is not walking away once you have set it.

Pick one person and one skill, and set a target slightly beyond what they managed last time. Say what you will do to help them get there. Check in before the deadline rather than after.