I didn't like how the weight of a trunk bag and rack changed the feel of my ride (a Dahon Hammerhead), so I went to a backpack. The backpack (Mission Workshop Vandal) felt much lighter when I added the waist belt to it.
I just came across a couple of blogs dedicated to "bikepacking," which is about packing the bike lighter than the more common rear rack/panniers setup. I think they have something there:
http://www.bikepacking.net/