I'd declare - and if in doubt, ask. Certainly, wearing something once or twice wouldn't exempt it from being declarable.

My instinct is that if you were gone a long time and bought something early on, using up a significant amount of its lifespan, it would be reduced in value or exempted from taxes and duty.

The safe way to deal with it is not to game the system - just disclose:

"Anything to declare?"
"Yes, here is a list of everything I've bought. On the bottom is $400 worth of clothing, but I bought those items 3 months ago and have worn them extensively during my trip overseas. I wasn't sure if they should be declared at full value or not."

Now you're safe - they'll do what they need to do, and you've made the disclosure.