Packing Light

I pride myself on how, the longer my trip is, the less I pack. Here is a picture of the tentative contents of my bag so far. For a 5 month trip, the bag currently comes in at less than 20 pounds. It is small, but it is still too heavy. (That's a checkbook in the corner as a reference for size.)
For those of you with experience in such matters, I am interested in your thoughts on things I "must" take and things I can do without.
I have a "steri-pen" to sterilize water.
I currently am thinking that the sleep sheet will be staying home. It is just too heavy.
3 pairs of socks or 4?
Hiking boots or running shoes, as my substantial footwear?
Do I really need my own full-blown towel, or is a cotton sarong much more sensible (and beach friendly) in hot weather?
Despite my best efforts, I am sure I will pack too much.