Hint: it's easier to keep limbs still if you can rest them against some rigid part in at least 2 dimensions, and from a comfortable position of the rest of the body.
E.g. a hand is easy to keep still if it rests on a mold of a hand (doesn't even need to be the same hand as long as it matches roughly in size). This also provides a reference for the person, so they know where to keep their hand.
My new favourite is a vacuum bag thing that you out around a limb (or baby) and then suck the air out, leaving a vacuum packed person.
Comfort (pillows and blanket), sandbags, sponges, straps, tape, coaxing, as-short-as-reasonable imaging protocols and a good explanation are all standard.
It usually isn’t direct movement of the part we are scanning that’s an issue. But things like shuffling hips, stretching arms and wriggling shoulders are a problem when resolution is something like 0.25mm in plane for your foot or knee.
E.g. a hand is easy to keep still if it rests on a mold of a hand (doesn't even need to be the same hand as long as it matches roughly in size). This also provides a reference for the person, so they know where to keep their hand.