Max out Vanguard and Crucible Marks: We know there will be new weapons available for sale in the NPC shops at the tower. It is also very likely these new weapons will have a higher max Attack than the current generation of legendary weapons. Be prepared to buy these on day one by having Vanguard and Crucible marks in reserve.
There is also speculation that the vendors will sell armor with a higher light level. For folks who have not yet made level 30 this will be very attractive.
Save Legendary and Exotic Engrams: There is good reason to believe that the new items from the expansion will be added to the loot pool that engram decoding pulls from. Its unlikely (but possible) that Bungie has a way to differentiate between engrams from pre-DLC release and post-DLC release. This means that the engrams you find now could decode to new items from the DLC after 12/9.
Save Postmaster Packages: Many think the contents of these are decided when you open them. That means that by waiting until 12/9 you could give yourself the possibility of new gear from the DLC dropping in those packages. Also, opening these packages is good for 5,000 XP. Saving as many as possible will help you level up your new weapons and armor more quickly.
Save Upgrade Materials: If there are going to be new weapons and armor then you'll need planetary and ascendant materials to upgrade them. Start saving now. Current legendary weapons might become somewhat obsolete if all of the new legendaries have a higher max attack stat. I would avoid upgrading anything but an exotic weapon or raid legendary right now. The raid weapons are exceptional enough so they'll probably still be worth having leveled to use until you get a chance to snag some of the raid legendaries from Crota's End.
Glimmer: Iron Banner 2.0 was pretty Gimmer intensive. It might be a good idea to bank Glimmer consumables prior to The Dark Below just in case. This one feels like less of a big deal to me. You could always go farm Exclusion Zone if you really needed Glimmer anyway.
TLW: Save your legendary and exotic engrams, vanguard and crucible marks, and upgrade materials until December 9th. Using those things now might be like buying an iPhone the day before Apple is scheduled to announce a new model.