How can I prevent a wobbly permanent tooth from falling out?

See a dentist. First question is why is it wobbly, from a bad bite or gum disease. Only a dentist can tell you why and then there might be ways to correct it.
Perio treatment. If your tooth is wobbly to the point that you fear it falling out, it is probably not salvagable and needs to be extracted. However, it is very uncommon for a tooth to just fall out. You need to see a dentist to determine the pathology around your tooth that is causing it to be loose. Often a mobile tooth can be treated with periodontal treatment to increase the bone level that hold the tooth in.