My keychain is a mini folding DMT that can be got anywhere for $10-$15. Very light and flat, goes with me everywhere, and there isn't a steel on earth I can't touch up with it--whether just pulling an edge straight or actually removing metal. A piece of 3M sandpaper with your leg as a backing works great to strop on, too. If I can remember to bring a knife along, no reason I can't remember to bring that along.
Of course, the secret is, don't let them get all that dull to begin with. If you let an edge really start to dull before you bring it back up to speed, you're likely going to be into serious metal removal because you've had to push very hard on it to keep it cutting, and this accelerates damage of rolled areas and other minor deformations.