Well, for one thing, I try to order from Canada-based stores when possible. (That’s why I love PinkCherry.ca so much! Kiss kiss!)
Another thing is, when I order from US-based (or international) retailers, I try to keep my order total under $100 when possible. I don’t know what the actual threshold is for when they start charging duties, but I do know that I’ve ordered ~$125 worth of stuff and been charged duties, but have received packages in the $80-90 range that didn’t charge me anything. So I’d estimate the threshold is probably in there somewhere.
That’s one of the things that’s great about ASOS, particularly: it doesn’t charge shipping, so there’s really no reason why you couldn’t just place two or three medium-sized orders instead of one large one. Saves you the extra cost when it arrives, and doesn’t really have a downside as far as I can see (other than the environmental issues associated with the one extra cardboard shipping box).