This can be one of the trickiest things to master in Naxx (and it was even FASTER @ L60), but it's cake once you get past the little annoyances like lag.
Not sure if you read my Naxx FAQ, but I posted a few animated samples of the dance that may help:
http://www.theorderls.com/belt/wm/wow/heigan.html - Flash movie showing the repeating pattern from "spot" 1 to 4.
http://www.warcraftmovies.com/movieview.php?id=30416 - Watch the Google stream, skip to 00:57. It's from L60 Heigan, but the general idea is exactly the same. VERY good tutorial.
http://img141.imageshack.us/my.php?image=heiganezmodewi4.swf - Another Flash movie, but it includes the Phase 2 "reset" 20 seconds into the film. Ignore the part about the teleported players, it doesn't occur in the new Naxx.
If you're not feeling very confident (or you want a stab at the Achievement), bring those colored smoke flares engineers use. Then have a ranged class quickly place them along the ground as shown here:
Thanks!