Easy sneak

How to Master Sneak Fast in Skyrim
In Skyrim, sneak is a very useful tool especially for two purposes. You can go the creeper toon that no one can see, or use the levels to apply to Magika or health instead of stamina (which is what an assasain/thief wants) and use the perks on other skills other than sneak! Here's how. (MAKE SURE YOU HAVE A DAGGER! or two)
1You need to have gained entrance to High Hrothgar on The Throat of the World. (this is achieved by getting further in story)
2Once you get done with the Greybeard's training in the Thu'um, stand at the main entrance.
3Go down the corridor to the right and sneak while doing so.
4Pull out those lethal daggers and you will see a Greybeard meditating at an altar.
5Go behind him, save and turn the difficulty to maximum (this way you may score more hits and not wait for him to recharge health as much)
6Beat on him. :)
7Once he either stands up or notices you, run and hide around the corner down the stairs. (if you have the Silent Roll perk for sneak use that)
8Repeat process until your sneak is lvl 100!