One of the most overlooked powerleveling techniques in Diablo 3 is with the proper and prodigious use of gems. Not everyone knows just how good you can get it when you use gems to increase the amount of experience gained, as well as a lot of other things like increasing the chance of getting D3 Gold and D3 Items. Of course, the main focus is on the experience gain, and gems can do quite a lot to give an extra boost with this. In order to do this, the key is in leveling your jeweler artisan, Covetous Shen.
Among the two current artisans, the jeweler is the more helpful one, unless Blizzard adds even more crafting options for the blacksmith. What you're looking for are Rubies, which give additional percentage of experience when slotted into a helmet. This is what lets you gain a whole lot of experience, especially early in the game. All you need is the highest level Ruby you have and a socketed helm, then a farming spot. Once you have all three, crank up Monster Power to your highest survivable level and start hording the experience points.
These bonuses can start as low as 5% additional experience points per kill with a chipped Ruby, which is actually plenty. You can then work your way up to a Radiant Star Ruby, which can be crafted by a level 10 jeweler. You can get an astonishing 31% bonus experience per kill when you use that, which is incredibly advantageous indeed. Even high level characters can get a good use from that, and they're most likely the once who can get access to Radiant Star Rubies.
Remember that you can unsocket gems from your equipment, so it means that you can reuse those gems as you upgrade your gear throughout the game. This makes socketed items very valuable in the game as they are essentially a requirement for creating the strongest characters in the game. They're also crucial for powerleveling, so get the best socketed items you can find for your character in order to make the most of everything. If you get more than what you need, you can always sell them in the auction house.
To understand just how much experience you can get from using gems, imagine that you have a Radiant Star Ruby and fighting against a big group of enemies at once. You take them out with AoE and other power abilities that are good against groups of enemies. You then get experience for killing each of them, as well as get experience bonus from feats like Massacre, then get additional 31% experience from each kill as well. That is a really big boost in experience gain that can help even level 60 characters who are increasing their Paragon level.
This could do even more in team play as you can have up to four characters leveling up really quickly with Rubies and Monster Power, as well as the team synergy, all working for you in gathering experience, as well as Diablo 3 Gold and Diablo 3 Items. That kind of powerleveling can be quite addictive. If that is what you're looking for in Powerleveling, then this is definitely a good thing to remember.