Something fun to try (it works and produced decent pak Choi for me).
If you buy it at the grocery store, after you use the leaves and petioles (leaf stems), the main plant stem is left. Put that in a plastic bag or container, place that somewhere inside out of the sun. It will start developing growing shoots and roots from the base of the stem. After that, bury the whole thing point up, just below soil surface.
Most times, it will start growing again. I did this early this year, got a really good pak choi. Plenty for a stir fry, beautiful tasty leaves and stems.