Rugby teams need to disorganise defences before they can expect to use the game breakers to the best effect. This means probing defences with big runners, or stretching defences by playing the ball wide.
However, the best piece of advice is to keep your back-line players on their feet and more essentially the game breakers. Don't let them go into rucks and mauls or take up crash balls. Let players work off them until the chances appear.