What can I do to help heal a broken rib from karate?

Rest. Rib injuries are pretty common in karate and MMA. Initially you may want to ice the sore ribs for 20 min at a time the first 3 days - most important time is right after the injury to limit swelling and bleeding. After that, it just takes time. We no longer recommend wrapping the chest with compression wraps because it will prevent you from taking full breaths and increase your risk of pneumonia.
Time. Really time and a good diet are all that help. It can cause pain for a few months, so be patient.