Plan to trim back purple hibiscus branches several times during the growing season. This is not an exact science, so don't worry if you forget or prune it back too much; it will survive, provided you are caring for its other needs.
Prune the branches back approximately 4 or 5 inches. Snip them off right above a healthy set of leaves, using a pair of garden clippers. The plant will branch out from there.
Cut back a purple hibiscus to the ground in the fall after it dies, if applicable. This simply cleans up the garden and helps prevent pests from making this area of your garden their home for the winter.