I think the trick is to find phrases that are authentic for YOU. Just like with compliments -- they have to be sincere. If you screwed up, and were wrong, then you own it with a true apology (not one of those lame "mistakes were made"/"I regret if you were offended," Washington-type ones -- a REAL one). And if not, then you validate with something that's value-neutral, yet sincere.
If "I'm really sorry you feel that way," or "That must have been hard for you" doesn't ring as "authentic" with the way you speak, then maybe a "God, I gotta imagine that would SUCK!" or something else that is 'you'.
I'd give you more, but those were all taught in "Validating 202," and I skipped most of the time cuz it was an 8am lecture hall.
