Apparently, it’s pretty simple- you need big players who have played for a while.
Paper (link to first page pdf)
How they won Rugby World Cup through height, mass and collective experience. Adrien Sedeaud, Andy Marc, Julien Schipman, Muriel Tafflet, Jean-Philippe Hager, Jean-François Toussaint. Br J Sports Med doi:10.1136/bjsports-2011-090506
Objectives To investigate the evolution of anthropometric characteristics in World Cup rugby players and identify elements associated with performance.
Design Age, weight and height were collected for 2692 World Cup rugby players as well as rankings in each World Cup, and collective experience of winners, finalists, semifinalists and quarter finalists in comparison to the rest of the competitors. Anthropometric parameters were compared according to age and position (back and forwards).
Results From 1987 to 2007, forwards and backs have become heavier by 6.63 and 6.68 kg and taller by 0.61 and 1.09 cm, respectively. The collective experience of the forwards’ pack is a value increasing with the final ranking attained, as well as the weight of forwards and the height of backs.
Conclusions For all Rugby World Cups, the highest performing teams have the tallest backs and heaviest forwards with the highest percentage of collective experience.