As some commenters have pointed out and something that I have failed to emphasize (I can't forgive myself for missing this), the foremost way in which you make gold in WoW is simply by completing all the quests in each zone.
As you can see, I have finally reached 10,000 gold from quest rewards. Since this achievement started counting after Wrath of the Lich King launched (please correct me if I'm wrong, my memory is fuzzy on this one), this means that at least 10,000 gold can be made in Northrend.
In my case, I have done all the quests in all zones except Icecrown and Storm Peaks where I'm about half-way through. The 10K amount also includes a lot of dailies.
Speaking of dailies, they are an additional source of income and should definitely be attempted when it's convenient to do so.
So there you have it: Blizzard made sure a lot of gold comes your way simply through regular questing. You most likely won't have 10K at level 77 when Cold Weather Flying becomes available but at least you'll have a head start. If you also combine this with 1-2 gathering professions, I don't see any reason why at 77 you won't be jumping on that epic flyer straight away.
Former WoW player, current Diablo 3 fanatic, I love the finer things in life such as PC games, beer and computers. While playing a good game is a spiritual experience, blogging about it is almost equally rewarding.