I use a DMT diafold diamond sharpener. Medium and fine grit.
Also have a Lansky coarse grit diamond flat.
They are both very light, and compact making them easy to carry in the pack.

thesurefire has a good idea regarding the sandpaper too. I usually
carry some 1200 grit as well, and have used the inside of my leather
belt to strop the blade after.
Seems to work all right, although some have suggested that if your pants
are made from some type of relatively coarse material like denim, you
can use that to strop a blade as well.