At home I have a series of stones from coarse to fine, used with oil. In the field, I carry a small, fairly fine stone and use whatever moisture is handy, (usually spit) to float it. The trick I've found is to sharpen the blade before it gets dull. If you have to hog a lot of metal off the edge with a coarse stone, you've waited too long. A couple of licks occasionally during use will keep the edge. Also I've found that a cardboard box makes a great ersatz strop. The grit that dulls your knife when you cut the cardboard, will sharpen your edge when used as a strop. You don't need a whole box, just a strip of cardboard out of it.