With Preparatory Shot power and Back Stabber feat, Heroic level PCs could constantly deal Dex mod + Int mod + 2d8 damage every round to targeted creature till they die? Are there better ways of getting SA and CA every round for optimum damage output with or without hybrid or multiclassing?
As a rogue (since thieves have sufficient tricks to basically get combat advantage every bloody round), using daggers, we can look here and discover: Cunning Stalker (CA on an enemy when they're not adjacent to another enemy) or Vicious Advantage (CA on a slowed or immobilized enemy). Both are trivial to achieve.
In terms of stacking extra damage on top of that, there are so many ways that I'm going to merely refer you to the guide linked above. For a fun time, use Riposte strike and be a thug for +Str with an interrupt of full damage + sneak attack + str if they attack you.
In a general sense, the best way to get sneak attacks is to have your friends powering you. If you have a friendly warlord, shaman, and ardent, you can get four times as many sneak attacks a round.