A Zara employee, Míriam Cava, has published a video with tricks and the reception has been such that she has added a second part with some more advice. She explains that she replenishes daily, but not everything, of course.
If you don't feel like waiting loaded with the clothes you want to try on until your turn, you can save time and book a fitting room through the website or the mobile application. The best hours are first and last, the Zara clerk is clear.