How do you raise your sneak?

You raise it by sneaking near an enemy (and not being caught obv )

Um ... sneak.

Seriously just use it. It goes up pretty fast.

Just...sneak (any time you're undetected in something's detection radius, the skill will start to increase). Or find someone to train you.

You'll raise it the best in combat situations, involving being in sneak mode, and remaining unseen. When you get successful melee/ranged sneak attack kills, you'll be levelling it awesomely.

Whats a easy to do it with a companion?

To clarify, does it have to be an enemy? Or any NPC?

As far as I can tell, its the former. Or rather, I noticed no skill up in sneak when skulking around Whiterun. But plenty of skill-ups when in a dungeon/near actual enemies. As a power leveler, it stinks, as it means I can't leave the game on and reliably skill up by sneaking into a wall.

i found that being detected then going into hiding raises it immensely! get detected then find a place to hide and wait till that eye shuts completely and you're almost guaranteed a level up. the key is not being able to be found. be out of reach then shoot an arrow, they'll look for you then can't find you so they give up and you're hidden again. wa-lah!

Start a dungeon with sneak mode on and don't turn it off. I was pretty shocked how fast it will level, but if you want to go even faster, make sure you're using the Thief Guardian Stone

I haven't tried this in Skyrim yet, but in Oblivion, I would just go to the local inn, go upstairs (when everyone was downstairs, and out of line-of-sight), and just crouched and walked around in circles. Because there is someone within range, but not seeing me, I am gaining experience in being stealthy. Just find a way to somehow do something else, like play gameboy while doing it, because it can be somewhat boring at the higher levels.

And i think it should pay off to spend one or two points in the first sneak skill as soon as possible, it lowers your chance on detection.

There's a khajit encampment right outside the first arch towards the entrance to Whiterun. The black khajit with armor can teach you sneak.

You can raise your sneak by:
1) Sneaking near an enemy when he hasn't detected you
2) Getting a successful sneak attack on an enemy/NPC

You won't increase your sneak if you're sneaking around a friendly NPC.

If you sneak through an entire dungeon and sneak attack guys one after another it will go up really really fast.

No they got rid of that. You only level from sneaking around enemies or killing while sneaking (which can be friend or foe).

You can sneak with your companion, they will sneak with you. However, I've had Lydia come out of sneak a few times to start unleashing arrows. So what I'll do is have her wait in a spot then I sneak around so she's not near me. Then she unleashes hell and I can get my sneak crit kills.

I've also put 2 perks into Sneak and bought a train from the Khajiit outside Whiterun. You will find it levels really quick after that - just have to use it! And as said previously, if you get spotted dont spaz out to a melee engagement; instead, move with a purpose to find a hide and get that detection indicator down. Boom! Level up sneak.

1) Go to a stable, Whiterun works fine.
2) Get behind horse, stealth.
3) Make sure you are hidden and not detected
4) Autorun into wall for an hour or two
5) Max stealth

I sneak into a dungeon. When enemies are in hearing distance I'll fire an arrow into the back of a room. If all goes right, they'll start looking for you, sneak goes up and they wont find you. I'm using the mage stone and my sneaking is still higher than my spell skills.

Sneak is like the easisest thing to level up... I am at around 50 without even specialising on it and im level 13...

Just sneak all the time near enemies...

Wow 50??? How did you get it so high? And level 13 too omg!!??