What are the best ways to prevent movement in a broken bone?

Immobilization. There are many ways to immobilize a broken bone. Most commonly is a cast, using a plaster or fiberglass cast to the entire length of the involved bone, including the joints above and below the break. Other ways include "internal fixation" where a metal plate or rod is placed either in or on the bone. Lastly is external fixation, where a bar is on the outside of the body attached w/ pins & screws.
Several ways. Most fractures are treated with a cast/splints. More severe fractures may need internal fixation.