I think a lot of newer pack-centric stories skip over the mechanics in favor of the drama, which is a shame. The traditional method involves intense meditation and anchoring to her mate's presence—it's a psychic tether, not just love. In 'Luna's Vow', for instance, she spends years mastering a mental sanctuary, a kind of mind palace where she can lock the wolf away even under the moon. It's grueling and often fails if she's emotionally compromised. I've read ones where the Luna's control is tied to her authority over the pack itself; if the pack respects her, their collective will stabilizes her. But if there's dissent, her control frays. Makes the political subplots actually matter to the supernatural struggle.
Honestly, the best portrayals show it as a constant, draining effort, not an on-off switch. She might channel the energy into other pack-link abilities instead of a full shift, like enhanced senses or healing. The full transformation is usually a last resort or a loss of control, which makes those scenes more impactful.