For field use, I carry a
Diamond Mini-Sharp Sharpener Fine Grit I really prefer to use "extra fine" grit for most regular sharpening, but the "fine" grit is good in case you also need to repair some damage.
I concur, DMT diamond stones are excellent. Perfect for a quick touch up and they work well with even the hardest of modern blade steels.
I prefer this model though:
http://theconsumerlink.com/DIAMONDMACHININGTECHNOLOGY/detail/TCL+FWFC/110 It's a bit bigger than the keychain size model, but it's faster to use and you get both a course and fine edge.