In my opinion, Zygor makes the best WoW leveling guides. They have been around for more than 10 years, which shows their commitment to maintaining and updating their guides to provide you with an optimized leveling path for any class and race, whether it’s Horde or Alliance. Zygor is an addon that includes a waypoint arrow and a separate window with instructions that, taken together, show you step-by-step how to level your character to 120, FAST. You’ll know exactly what to do at all times. It’s also very easy to jump in at any level; you don’t have to start exactly at level 1 or 110 or anything. It’s a bit more expensive than the other options, but I’d say it’s definitely worth it.
Buffs. Food/Flasks/Scrolls/Potions/Elixirs are a small, but noticeable, buff to your killing speed and survivability. If you have a max level character that can already supply you with cooking or alchemy, you'll be very well off, because the AH is a cesspool for this. I can't really give you specific foods or potions to look for, since each AH is going to have different supplies. Generally speaking, lower level foods are going to only last 15-20 minutes until you can buy the more powerful versions at 60+. Look for anything that adds versatility or your primary stat, and buy stacks for each level range(1-60/70/80/90/100)
Something to keep in mind as well is that this will make enchanting materials more valuable, since it is likely most players will now simply scrap their boe greens instead of getting them disenchanted. Also redundant dungeon/raid gear can be scrapped rather than traded to the (guild) enchanter in the run. Less stuff gets disenchanted, meaning there will be less ecnhanting materials, meaning their value will rise. All the more reason to DE rather than scrap as an ecnhanter.