In my opinion backstabbing is best used if the opponent has been frozen, blinded etc. beforehand. Also I usually terminate the turn with either walk in shadows or tactical retreat, or throw a smoke grenade to turn "invisible". Also, the killing blow is very handy. Try also to equip daggers which have a chance to freeze, knockdown or stun the opponents - 2 daggers with 20% chance each to set the frozen status is really helpful. Put attribute points into speed and find gear which gives you movement bonus (at level 15, I had a rogue who could walk 7m with one AP and could cross the battlefield almost 3 times in one turn). As a general rule: Do not remain standing behind the opponent after backstabbing without taking any additional precautions.
