This, one of the signature skills of the "rogue" class nor causes all the damage that should do nor triggers as much as it should.
While it's true that sneak attack is a powerful skill, with a humble D6 of hp and other weaknesses it's one of the few reliable assets this class has and something that can truly differentiate it from other classes.
What I propose is modifying the backstab/circle stab skills so;
Both skills are able to do up to 10D6 at GM for each sneak attack, what would mean that at GM they should be able to do as low as 6hp extra damage and up to 60hp damage.
If you are hiding and you attack you should always trigger the backstab attack (as long you hit your target), if an enemy is unaware of your presence and you attack him he is caught surprised one of the scenarios in which you are eligible to sneak attack a target.
If you are attacking a mob that is attacking anyone else in your party, all your attacks should be "circle stabs" since you are flanking them, you should not be able to circle stab someone who is attacking you unless he is in a situation in which he can't defend himself effectively (most likely because magic affects).
It's true that sneak attack is powerful, but so are high level spells, twined/maximized/empowered spells, not to mention save-or-die spells, or all the potential that can cause a fighter with high strength with a two-handed weapon and the power-attack feat, also sneak attack has the added weakness that it can not harm undead, constructs, oozes, plants, and incorporeal creatures whereas other attacks can do.