Obviously, from the player and their various coaches, physios and the like, a lot of work and effort and dedication. But that wasn't quite what I meant.
Rather, if you look at the sizes of player pools available why are some hitting way over their weight? Looking at the senior males numbers, England have 166,000, France 110,000, South Africa 109,000. Although there isn't a convenient sort routine, Australia at about 40k, then USA at 35k, beat out a cluster of New Zealand, Ireland and Wales at around 25k, then it's a step down to Scotland and Italy (around 12k) and then the rest of the world. (Note that Argentina don't report senior male numbers but have total males > Australia, so should possibly be up above Australia for senior males too.)
So why are the USA so low in the world rankings (17 at the moment). Although England and South Africa have been world number 1, I don't think France has, and New Zealand are, however much you might not want to hear it, the most successful side since rankings were introduced, with the highest ever rating and by far the longest possession of the number 1 spot despite being 6th or 7th in terms of total size?
I suspect the answer is largely in the mind. Or perhaps more accurately in the experience. While you can argue who the fittest (in a physical sense) individual is, certainly down to Italy and Scotland in the rankings and sizes, you don't see sides routinely win because of better fitness - although you can see sides in the lead stretch that with a combination of mental and physical willingness. But rugby, perhaps more than any other team sport, is about the brain. Making the choices to join the ruck or stand off, pass, off-load, take the tackle and so on. They are all essential choices in rugby that every player makes throughout the game. This is why someone like Brian O'Driscoll is still a very good player. He doesn't have the speed and agility that made him dangerous as a younger man, but he knows now much better how to apply himself, when to find that extra burst of power or force his old bones to make that swerve. (OK, I'm making him sound like he's only walking with a zimmer frame and he's still fit enough for 80 minutes of international rugby but it is clearly getting harder and harder for him to stay at the top level physically.)
And here, I think, national culture and club structures play a vital role. England and France play club rugby not to lose and with a lot of foreign players. The former Celtic league (now with Italy of course), Super Rugby and the like, play largely with home-grown players and because there is no penalty beyond pride for losing, they play to win. In addition, all these countries have relatively few top-flight sides - four or five typically, but because they're largely home-grown talent you also have a solid pool of players at the highest level not dissimilar to the pools of the biggest countries' unions. Plus you have the benefit of being able to choose more combinations of players used to playing together at club level as well - Nonu and Smith (both Hurricanes) beat out any combination with SBW in big matches due, I largely suspect, to familiarity. When Carter and Slade went down, Cruden (also a Hurricane last year) slotted in comfortably between Weepu (ALSO a Hurricane) and Nonu - how much did that familiarity help them all?
Add to that in New Zealand well coached, structured and scouted rugby from at least the age of seven, and a system (NPC) under Super Rugby level that mixes new talent, talent at current Super Rugby but not quite All Black level and retired All Blacks for a year or two, and you also get a system that spreads the skills and experience into the youngsters.
And that, I suspect, is the true magic. All those things that we're used to seeing the All Blacks do so well, year after year, game after game, are down to the fact that when as a brash youngster of 22 or so they take they field, they play with an extra chunk of year's experience behind them of playing top-flight competitive age-group rugby. They're making the smart choices of the BOD of today, with the physical exuberance of the BOD of a decade ago.
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