I think Edisim said it well. What Nintendo needs to do is sell their system before it's out. Build the hype. Let's face it, they don't need to work to sell the system to Nintendophiles, we all know we're gonna buy one anyway. They need to work to sell the system to the casual gamers, and that means they need to do a few things.
1) They gotta get solid third party support, and come out with a few big games of their own that will sell to the casual gamer. A big racing game here (with more mass-market appeal than Mario Kart), a snowboarding game there (improve the 1080 franchise), and a few other things to draw in the casual gamers.
2) They should work that cool-ass Zelda demo from SpaceWorld into a game - something dark and awesome. I loved Wind Waker, it was a fantastic game, but unfortunately, the fact that it kicked ass didn't draw people in - they were first and foremost turned off by the graphics. If the game was identical to Wind Waker with 3D polygonal graphics that pushed the Cube to it's limit, it would have attracted many more people (and maybe lost a bit of it's charm). If there's one franchise Nintendo has that they could make look cool enough to appeal to the mass market without touching the essence of the game, it's probably Zelda.
3) They have to come up with something to rival X-Box live. A lot of games are getting sold based on X-Box live playability. X-Box live was set up really well, and Nintendo has plenty of games that would work well on a similar format. For starters, there are all of the sports games and stuff. Then there's Mario Kart, Smash Bros, 1080, Mario Party (maybe, if they can do something more original with the series), PSO, and maybe a few things they could either develop themselves or have other studios develop for the purpose of using such an online system.
4) Advertise the hell out of everything. Commercials on TV. Billboards. Trailers at movie theatres. Put slips advertising other games in EVERYTHING (like that RPG booklet they had in Mario & Luigi, that was pretty cool). They've got this whole system planned where buying stuff gets you points to get free stuff (or however it works) - if that comes to fruition, advertise the hell out of that, too. At the end of every ad, do something like "Don't forget to register it online to get points that will get you some KICKASS FREE STUFF!!!" Advertise it so much that it's burned into the brains of everyone who owns a TV.
Of course, that's just what I think they need to do. My opinions don't really matter much until I have a marketing degree, which I don't.