Well I usually just match whatever looks nice for the room I'm going for. For example in my basement the main furniture is all Alpine but it doesn't look like an 'Alpine Room' because I don't have the full set in there and I have two couches. Furthermore it's customized with a pattern, the wall and floor don't match and I have lot's of other items. (A tv, zodiac and event knick-nacks, things like that).
In another room I have mixed classic with some regal furniture. Honestly though my favourite things to match with series furniture is non series furniture. The cream sofa, the common chair, RV items, etc. etc. The possibilities are endless.

Duplicates and leaving out bits of the set is a good way to go.
I also think it looks best when the room has a function. This function can be one of your standard rooms in a house but it doesn't have to be. You could make anything. The basement I have described is meant to be cozy hangout. If a room looks lived in/used and not like a display I feel like I have succeeded.
