I've got a character on level 70 with maxed out warrior skill and almost maxed out thief skill. I've been sneaking in every dungeon crawl but my sneak is only at 92 after 174 hours of playthrough. I want to now just start leveling Mage skills to get to 81.

Any suggestions to level up sneak quickly will be appreciated.

I'm not nearly as high in sneak as you (just started her yesterday and I've been playing on my other characters), but I got to 50 in less than a couple hours gameplay by sneaking around Darkwater Crossing a lot. Got in a corner where the guards couldn't see me by the mine and just walked into the wall. Just like I would do back in MW/OB days

Have you invested any perks on Sneak tree? Maybe it levels up quicker this way. I've yet to invest any perks on this tree.

I forget where I read this, but I think some tooltip said that the sneak skill only increases if you're sneaking around something that would otherwise see you. And from successful sneak attacks.

Okay, so I just read on wikia that you can walk behind a bartender at any tavern and keep sneak walking backwards to raise sneak quickly. I'll give this a go and see what happens.

One of the many tips I've read also said to sneak behind one of the Greybeards and repeatedly attack them with something that does little damage, and waiting a few seconds in between attacks so they don't get mad at you. Another one is to take a quest that requires you to meet an essential NPC somewhere (you can do this in the tutorial with Hadvar/Ralof, or later with Delphine, Mercer Frey, and various other quest givers), sneak around them or fire arrows from a distance while in sneak mode.

Make sure you're always wearing sneak fortified armor/jewelry. Training from a Master level (Delvin at Thieves Guild) stops at 90, so you can't level up by training beyond this. This leaves 3 other choices. Improve your skill:

1. from reading sneak books you encounter in dugeons, towns etc. or

2. from successfully sneaking past enemies in combat/dungeon/bandit camp situations. Also past bandits/wild enemies you encounter on the roads. Sneaking past a bandits, sabre cat, giant, bear, Falmer, Ghosts, Wisps, and Dragur seemed to work really well for me. Or

3. from trespassing. Try sneaking inside NPC houses at night/early morning when they're still inside their homes and their doors are locked. In particular, sneaking around in an NPC bedroom while they're sleeping on their bed seems to raise the skill faster.

I can hardly think of another skill that non-requires power-leveling than Sneak.

Just... Be Sneaky. You'll get there. I've maxed it out more than once without even taking the Thief stone bonus.

Sitting still and waiting while the "eye" is partially open and gradually closes is particularly good for raising Sneak. But that requires... sitting still. And waiting.

A tall order for some folks, I guess.

I heard there was an early dungeon with a spider and a wall you can sneak behind and the spider can't git you. But unfortunately I do not remember the details. Anyone else?

Find a crowded spot like just outside the Inn in Solitude or a Guard barracks. Face a wall, go into sneak mode and press C (or whatever the Auto Walk button is if you're on Console). Go to bed, watch a movie, get drunk or whatever. Come back and have 100 sneak. Easy.

Actually the easiest way to lvl sneak is to get an iron dagger, go to High Hrothgar, and continuously sneak attack the Grey Beards there. (Easy way to do this is sneak attack with a dagger in one hand and continuously heal with the other hand).
Its a pretty rapid way to lvl your sneak. Just be sure of a few things:

1. You are hidden when you attack
2. You do not and I repeat do not attack a grey beard until they hit zero hit points (they are essential NPCs and cannot be killed. Also when you force them on their knees they auto aggro to you).
3. Reason for this is because Grey beards know the Iceform shout and sometimes will iceform you and leave you there for good. When this happens its literally impossible to do anything because the iceform shout THEY do doesn't release you until they hurt you again with something else to melt the ice.

Okay, so I finally got there.

http://cloud.steampowered.com/ugc/649875346815488167/6BF860A45489C36C4D2EB15FD1276E2EC8020595/

Felt like I was being punished for being dunce of the class from the Bards college.