Best way to avoid damage playing Rogue in Dragon Age II

In addition to Z's answer, one of the Rogue's biggest benefit to a group is helping the tank (term used loosely at times, you can run parties with no true tanks) get the focus of enemies. Your Scoundrel tree is perfect for this:

• Armistice will force enemies around the selected friendly target to forget they exist, going after the 2nd on threat instead. You can use it on yourself, especially effective as melee when you're in the middle of everything.
• Back to Back will teleport you out of harm's way, but not make you lose threat. Effective to kite slowed enemies, maybe in concert with Miasmic Flask or Fatiguing Fog to let her kite more without getting hit.
• Goad is the opposite of Armistice, it forces enemies around the friendly target to target them (the friendly) instead. Sort of a forced AoE taunt.

Additionally you can use your shadow to escape from view unharmed. I've never went down that path myself though to comment on its effectiveness. I prefer playing with the threat of enemies instead.

Edit: Another really useful tip, sort of related to this, is to use your abilities as little as possible to survive. Rogues get their damage from auto-attacking, and using abilities interrupts their sustained DPS. Nothing wrong with stunning everything or redirecting focus if needed, but if you're in no danger just sit behind an enemy and kill it, you'll take a lot less damage the less they live (especially in a large cluster, kill 1-2 high damage squishy targets, like mages, and it suddenly becomes a lot easier to manage).