There was a cheesy way (dunno if it was patched) to get it high. If your sneak is already super high, say 100 with all the perks, then you can let yourself get caught in Riften. Or there is a guy in the Riften jail. He is behind bars and you can gather up objects from the jail and give them to him and take them over and over, periodically walking a loop to avoid a patrolling guard, but it only works if your sneak is super high. The reason this works is that the guy in jail cant report you since you are already in jail, and he cant attack you. So as long as the guards dont see you, you can just do it over and over.
Its most likely an exploit, in that it probably didnt occur to the devs that no repercussions means endless practice. But I figured i'd let you decide for yourself. And I have no idea if it was patched. I'm still playing on patch 1.1 in offline mode, since I never wanted the bugged 1.2, and havent gone back online to try any subsequent patches.
I did this for over an hour last night. My sneak is 100+ so I didn't have any problems pickpocketing Sibbi Black-Briar. The guards would just walk right past me.
The thing I don't get is what to do after I've gotten to level 100 pickpocket. I got my gear and sneaked out of the jail but the guards in Riften continue to attack me- no option to surrender or pay. My weapon is sheathed. Not sure what to do now. I ended up running out of town and then fast traveling to thieves guild. Entered the flaggon and moments later a guard showed up. I didn't have to do anything as the other guild members dispatched him quickly. No option to clear bounty like in Oblivion

I even tried going back to my cell and sleeping but since the guards discovered that I'd escaped I got the message 'you can't sleep while enemies are nearby'. What now?
patch is 1.3.5 on xbox