Hi All, A question I find useful after every trip "Did I use ___item" If after several trips I havn't needed it consider leaving it home.
To some extent this is useful. However, I've found that there IS core gear, such as rain gear. I've been on 10-day trips where there's no rain. But, you know what happens at 8,000 feet- that 70-deggree summer day drops to 40, lightening acts up, and suddenly everyone without raingear is hypothermic (happened before). So I carry it.
On the other hand, I've found that 3 pairs of underwear and socks, and 1 pair of pants, is plenty for that time frame(oh, don't fret. I run around in boxers while my pants dry)
In terms of a day-pack, food is usually the item I've found most useless, then a fixed blade knife.