I mostly did what Gurgthock did, except I'm leveling a mage.
I did the whole "white items" with super enchants, I had a level 1 dagger with +30 spell damage, pants with 25 spell damage kit, gloves with +20 fire damage (old-world pre-patch enchant, not sure that's possible anymore.). Obviously having 75+ fire damage at low levels lets you destroy things in almost one hit.
I kept up the twinking and always looked ahead, getting fiery wrath items and maximizing enchants and such. I've stayed around the same 350ish fire damage since my mid 30s. Once I talented Combustion, whenever it was up I'd be able to chain 1-shot mobs until the buff was gone.
A lot of people told me frost was more effective for leveling as a Mage, but I found being able to 1 shot mobs was a pretty quick way to go about killing things.
Twinking gear-wise costs a bit of money but really does drastically speed up your kill rate, and thus leveling rate. Most people who are leveling alts probably have a fair bit of money, anyway.
Anyone who's leveling a caster up and is obviously going Tailoring, I'd recommend starting the grind for the Elementals you need for whatever tailor spec, and grinding them until you ding 70, doing as few quests as possible. This way, you get the primals you need for your gear, and save a ton of Outlands quests for the massive gold it will give you doing them at 70. Probably save you a good 6-8k gold doing that, as opposed to leveling/questing all the way to 70, then buying the majority of your mats for your gear.
For casters, Adept's Elixir is usable at level 50, and that's just an extremely good elixir, due to the critical rating. You can use something like Mageblood with it, or Draenic Wisdom, both are really nice because they massively reduce downtime. Wizard Oil + Food is also great. Whenever I was in an area where I'd be grinding for a while, or, doing an instance, I'd obviously buff myself up to the fullest. It was hilarious to be lobbing fireballs more powerful than BWL era mages.
Also, don't overlook Jewel crafting items! There's tons of Necklaces and Rings that are extremely good, and aren't "search friendly" on the Auction House, or whatnot. Find a JC friend, ask them what their good lowbie stuff is. Or scour a JC tradeskill / recipe page somewhere. The necks that have "uses" that greatly buff you + your parties abilities for a period of time are amazing for their level.
This is my mage:
The World of Warcraft Armory
I stopped min/maxing every single gear slot upgrade because some of them were annoying instance drops that I didn't really want to run 5+ times for a chance at an upgrade, and instead would rather use that time doing my tradeskills/leveling!
(Oh yeah, you should also really skill your tailoring ASAP if you're a caster, that way you can start doing your Cloth transmutes as soon as possible.)
For outworld questing zones, there's definitely an excess of zones.
The following are zones you can completely skip:
Ashenvale
Wetlands
Thousand Needles
Stranglethorn Vale - I only did the Pirate chains from Booty Bay, because everything is so close together and easy, and very little cross-faction competition.
Swamp of Sorrows
Desolace - probably. I did a few quests here and realized the time I spent running back and forth between quest givers and the miles of land stretching between them with no mount, was probably a massively inefficient waste of time.
Un'goro Crater
Searing Gorge - I was only able to complete like 3 quests here and realized that while they were green, they were already being flagged as "low level" and giving me bad experience.
Silithus
Winterspring (Note: You do want to do the chain that ends in killing High Chief Winterfall, that's all though. The rewards are super, super, super good)
I skipped all of the above zones unless noted otherwise. Basically, any zone that I felt was kind of a pain in the ass terrain-wise, or I wasn't familiar with, I decided to go without. And it turns out there's still plenty of questing to be done, mostly because Dustwallow was hugely buffed, and covers a huge leveling range and duration, highly recommended.
For a long time while leveling I tried to use every zone and zigzag all over the world and get the most out of quests before they went gray, but that really proved impossible and not worth the time. I know the last time I leveled an alt you could easily run out of quests, but it's the polar opposite now, so pick your favorite zones and stick to those only.
I decided to not skip WPL/EPL because I wanted the Argent Dawn rep, and if your server is the type to run Naxxramas pugs sometimes, you may want that reputation on your alt incase you ever want to experience some of that old world content on a different character.