I like those folding dmt stones. They do a good job.
If you aren't far from your truck or jeep or whatever, you could always sharpen it up on the edge of the window. I do this if I need to touch something up at work and can't be bothered to dig out a stone. The edge it gives is very good and the glass cuts quite fast.
I am not a big fan of the sandpaper idea (although sandpaper does work well) just because if you find yourself having to take a nick out of anything, you are stuck hunting up a good rock to do the big grinding. I like the dmt stones because they are very cheap, very easy to use, last forever, and you can take off a ton of metal in a short time if you like.
Hopefully you don't wear jeans in the bush but I often do at work, and sometimes in the bush as well if I have gotten lazy or cocky - they strop quite well. If you wear canvas pants (which again I often do at work, and in the bush, and particularly when working in the bush, even though I know it's not a terrific idea) they strop really well.