I've come across an easy way to boost Sneak that takes a minute to set up, and then can be left for hours as your character continuously levels their Sneak while you shop, eat, sleep, etc. Go to Jorrvaskr (the Companion's Hall in Whiterun). Enter the living quarters (stairway to the basement is to the far left of the entrance). At the base of the stairs, turn right and go down the hallway. Take the first left, and then the bedroom on the right. Position your character on the chest at the foot of the bed, angled towards the bed. Enter Sneak and AutoRun mode (verify that your character is positioned so that he doesn't move) That's it! Skjor (the owner of the bedroom) will faithfully get up each morning and return to bed each evening, despite the fact that he sees you running on the spot like a buffoon in his bedroom! The advantage of power-sneaking in this fashion is that the hall is easily accessible (the most you might require is admittance to the Companions). While my test was done with Skjor, I suspect that it would work on the other NPCs in that guild as well. The disadvantage is that it is slow (you are in effect only leveling while the NPC is present; about a third of the time); it took about 8 hours of PC time to go from Sneak 75 to 95. Still, if you're not in a hurry it's a nice way to have your character progress at something when you're not playing the game.