Endurance on gear, basically. You get a little bump in health at each level increase, but the majority of your hp will come from the endurance on your gear. The key is modifiable gear, as in orange gear as you're leveling, and/or end-game purple gear with mod slots. Keep adding mods that favor endurance over your class skill, and you'll get the high hp numbers.
Note that very high hp is more useful for PvP than PvE (IMHO); for general questing, flashpoints, operations, etc, you'd want decent hp but also higher levels of your class stat so you can do more damage/healing/etc.