Legend has it than when New Zealanders teach children how to play rugby, they spend the first six hours of instruction teaching them how to stand properly. Seeing as New Zealand's national rugby club is currently the top-ranked in the entire world, maybe focusing on body position when learning rugby is important.
So we'll start with the most basic of the basic types of body positioning: how to stand when running with the ball, and how to stand when carrying the ball into a tackle.
Running with the ball is the basic building block of rugby, the act that separates it from shudder soccer. Running with the ball effectively is a question of size, speed, field vision, and training; a lot of effective running can't be taught, so we won't try.
Running with the ball using correct form won't make you faster or stronger, but it will make you a more dangerous rugby player in another way: it creates options and reduces the amount of information the person trying to tackle you has about what you intend to do.