Improved PW:S is a very wise investment when power leveling your character. Also improved flash heal (70% chance to not be interrupted) is a good thing to have if you want your mage to be able to survive while you are getting hit, however it is not necessary if your mage is doing his job. This strategy has been tested by many people and myself so it absolutely works great! On the down side this strategy only works with mages and priests. A group of a mage and a priest is the ideal way for this strategy to work for power leveling.

 
If the priest is getting hit you are both dead since his heals will be interrupted. Make sure you move around to keep aggro on all monsters for power leveling. Once mage gets instant arcane explosion then it is off to the races by killing groups of monsters slightly lower level than the mage/priest and be able to kill 10-20 of the mobs at once using AOE spells from the mage and the priest's flash heal spell but can also do it without the flash heal spell to some degree.

 

Also remember that you can combo frost nova with cone of cold for some massive damage if you are frost specced! Frost nova will also prevent mobs from running when they get low hp. Pretty much any build will do, just make sure that you have improved arcane explosion. If you go frost make sure to get improved crit % of cone of cold as well as lower the cooldown on frost nova. If you go fire then combustion is nice for that initial bang. Use frost nova to root monsters so that you can AOE them without getting damaged as much about power leveling.