First of all, I know these methods are nothing new, but the perspective I take does make a difference.
This is for solo non-botters, meaning you're not going the ZK route, and you're actually going to be sitting at your keyboard.
I've seen quite a few PL spots that are good, however, in my opinion, for the monotony involved, they're just not worth it; also, I believe they are slower.
For 50 - 60, I start off by getting the WP for A3 Q1, siege of bastion's keep; the difference is, I do not kill the first wave of mobs on the steps and leave the game, for me, it gets really old really fast, and you're pulling in way under 100k XP per run, it just gets so mind numbing I can't stand it (especially after doing it on multiple toons).
What I do, is clear the initial wave, and then clear up to the first signal fire (from the checkpoint, go up stairs, to the right, up the stairs, down the stairs, left, up stairs to signal fire).
Doing this, you still gain XP at a very fast rate (I would argue faster than the quick repetitions of just killing the first wave on the first stairs), you also usually encounter at least 1 - 2 boss packs, which give great XP. I do not loot anything but rare drops to keep from going to town. I do this from 50 to 56.
Once I hit 56, I get the WP for the final A3 quest, on the "Kill Azmodan" part of the quest, and clear the Core of Arreat. Whether or not you kill Azmodan is up to you - personally I don't bother killing Azmodan, but what I do like to do is get the checkpoint at the end of the core, then leave and resume and farm it backwards - simply clearing the area provides usually 2 - 3 boss packs, tons of very easy kills (huge waves of crawlers which are the ultimate "trash" mob in this game), and you will run into treasure goblins on a regular basis (always nice). The other nice thing is that this area is fairly packed, the run takes very little time, and is way less monotonous than killing Iskatu, shadows in the armory, the first wave in siege of bastion's keep, or 3 wretched mothers, over and over again.
Yes, the content is a little more challenging, but this late in the game, it shouldn't be an issue to get good enough gear to /roflstomp this content at 50-60. Level requirement reduced items are definitely your friend.
So as the title suggested, if you're looking to solo powerlevel, aren't botting, and you're tired of your eyes bleeding, give this a whirl!
Video Demonstration as requested: http://www.youtube.com/watch?v=yySxqnDNyvc