I've found that the key to taking a character from level 1 to level 60 quickly involves 3 main factors; Experience (duh), Damage, and Movement. I’ll cover Experience last since it’s the most complex. Using these basic guidelines, you should be at level 60 by the end of Act I Hell. Feel free to add anything I've missed.
1. Don’t spend a lot of money on gear. It will become obsolete within 30-60 minutes of game time.
2. Always get gear with a high +Exp on kill, even if it’s a lowly blue item.
3. If you’re smart with your purchases, you should be able to sell everything back to the AH for more than you paid, even after factoring in the 15% loss.
4. Remember Rule #1. That great L17 item will be junk in an hour. YOU DON’T NEED GREAT GEAR!
The faster you can kill the monsters, and the more of them you can kill in a short time period, the quicker you’ll gain experience. It also allows you to move through the maps faster since you’re not wasting time slugging it out with some pesky Goatman.
The formula for damage is very simple. As soon as you can, buy a socketed weapon and put a giant Ruby in it. Don’t fiddle around with something you find or have in your stash, go to the GAH and buy a Flawless Star or Radiant Star. You’ll use it all the way up to L60, so there’s no point in going cheap.
At L9, Genzaniku (legendary axe) is available, but is usually grossly overpriced (20M+).
At L11, The Barber (legendary dagger) is available, but again, isn’t worth the money (40M+).
At L15, regular weapons are available with a socket. This is when you buy one.
As you progress through the levels, keep upgrading your weapon to increase its +Exp bonus. You don’t need to do it every time a better +Exp becomes available, but it doesn’t hurt to check. The quality of the weapon itself is really not that important because Giant Ruby is going to pulverize pretty much everything in sight regardless. But keep upgrading to higher damage (every little bit helps).
Around L42, you’ll start to see L60 items that have a Reduced Level Requirement. These will generally be a huge upgrade over what you have, but they can be very expensive, and you generally won’t need one just yet (Giant Ruby is still going to be kicking !@#$). Keep looking though as you progress. Eventually you’ll find a reasonably priced upgrade. It doesn’t have to have a socket, but don’t sell that Ruby just yet. Going with an L60 item means you lose the +Exp bonus off your weapon, but by that time you’ll have other gear that will more than make up for that loss.
Moving quickly through each map is critical. You’re wasting time if you’re slower. Here are the levels and the available movement speeds on Boots.
L1 = 5%, L3 = 7%, L10 = 9%, L21 = 11%
At L33, Slave Bonds (legendary bracers) become available, and they add an additional 11% movement speed. Buy one as soon as you can.
At this point it’s just a matter of tweaking what you have. Movement speed of 12% doesn’t appear until L42 when the L60 items start appearing, but you won’t notice another 1%, so don’t buy them because you’ll lose the +Exp bonus.
Obviously the key element in leveling up. There are 3 ways to increase your Experience bonuses; Per Kill, Bonus %, and how you play the game.
Gear #1: +Exp per Kill
Every piece of gear you have should have +Exp per kill. It should be the single most important affix you look for when upgrading. Don’t worry about how much it will impact your Damage because Giant Ruby is going to take care of that. By the end of Act III Normal, you should have more than +200 Exp per kill in total on your character. Don’t go crazy on the purchase prices if you’re on a budget, but don’t let the bonuses lag either. As you go through the Arreat Crater, those swarms of bugs will give you huge experience.
At L21, look for Born’s Impunity (set shoulders) and Born’s Heart of Steel (set chest). Individually, they’ll have +10-12 Exp per kill, but with 2 items, you gain a +20 Exp bonus. That’s +40-44 between them, which you won’t be able to match with individual pieces until L39 or so. These are not cheap, but you should be able to find each one for under a million. They’re worth it, and you can easily sell them when you’re done. There’s a 3rd piece to that set (Born’s Searing Spite), but it’s a sword without a socket, so don’t bother.
Items will top out in the 22-24 +Exp range, but anything over 20 is excellent.
Gear #2: Exp% Bonus
At L1, if you have access to a Hellfire Ring, regardless of its main stat, put it on your character. The 35% Bonus is huge, and it very likely has stats that dwarf anything you’ll find on an item (huge Vit, bonus dmg, bonus armor, etc.).
At L14, Leorics Signet (legendary ring) is available. While it does add anywhere from 20-30% Bonus Exp, it’s generally very expensive (20M minimum, with many over 100M). Purchase one if you want to, but I think they’re grossly overpriced. If you have multiple characters you’re leveling, it may be worth buying.
At L15, socketed helms become available. Buy one and put a big Ruby in it. If you want to spend the money, buy another Flawless or Radiant Star. Like the one in the weapon, you’ll use it all the way to L60. (note: at L11, the WD has access to Split Tusk, a legendary mask with a socket).
At L23, Cain’s Fate becomes available (set with boots, pants, gloves, helm). All of them will have +14 Exp, but with 3 items, you gain a +30% experience bonus. The pants might have 2 sockets, which never hurts. Buy 3 of the set items as soon as possible. You’ll use them all the way to L60.
MP Exp% Bonus
This is very simple, play Normal, Nightmare, and Hell on MP 10. You’ll be doing more than enough damage with Giant Ruby to make it through pretty much unscathed.
Game Play Bonuses
This is the creative part of fast leveling. You get bonuses for destroying at least 6 items in a very short time period (more items = more bonus). You get bonuses for killing at least 6 monsters with one attack (AoE damage is especially helpful with this). And you get bonuses for killing at least 10 monsters in one continuous attack (just keep firing!).
The trick with items is they don’t have to be just the usual barrels and crates. Tables, chairs, tombstones, anything you can destroy will get you a bonus. I regularly used Disintegrate with my Wiz, or Grasp/Acid Cloud with my WD to destroy a room full of beds and tables. Sweeping Wind or Tempest Rush work well too for a Monk. If you do have two clusters of barrels/crates, position yourself between them and take them out together.
The monster bonus is fun too. You’ll learn when to launch your attack for the biggest effect. You can get bonuses in the thousands for a really big kill. Again, the Arreat Crater is awesome for this. Acid Cloud dropped on a big group of scorpions and they just melt.
The last item relates to your Follower of choice. Just like your character, you should equip them with +Exp items, and even a spare Hellfire if you have one. You will earn 20% of whatever bonuses they have. It may not sound like much, but it’s free experience.
In Normal mode, at the end of Act III on MP10, with a Hellfire and +Exp gear in every slot, you should be seeing a +Exp per kill of over 200 (my WD is currently at +220), and a Bonus % approaching 300%. A 1000 bonus Exp for every 5 kills is huge.
As you progress past L40 in Nightmare, the +Exp per kill starts to have diminishing returns. There’s simply too much Exp needed for each level. You can start to replace items with lower +Exp bonuses if they’ll give you a big advantage in another area (e.g. extra life, a shield with a much higher Crit Chance), but I wouldn’t eliminate the +Exp bonus completely unless you’re equipping an L60 item. However, you should NOT under any circumstances replace any of the Cain’s Fate items until you get to L60. They are more than adequate for whatever you face, and the 30% bonus is too much to pass up.
This is not the definitive source on fast leveling. It’s just what I’ve experienced recently as I’ve taken a monk, a wizard, and now a witch doctor from single digits to L60 (WD isn’t quite there yet). I already have an L60(69) DH, and I didn’t want to waste any time getting these others up to Inferno.