Keeping a small cheap stone, rod etc in your pack is simple, however there are several ways to improvise and there would be no shortage of alternate ways to sharpen your knife or maintain the edge if you needed to.  I have used the edge of a car window quite a bit and the bottom edge of a glass mug.  My grandfather used to use the bottom ring edge of a plate or cup and would strop his knives on his belt, sling or boot when in the field. 

Various types of rock and yes even concrete works well but I suggest doing it wet.  I run a chunk of concrete through my tile saw to bring back the edge on diamond blades for tile work and concrete can do the same for putting an edge on a knife.  You can also use a simple steel rod (not a sharpening rod) or the spine edge of another knife.  There are many ways to sharpen, hone and strop a knife.  While I strop on a leather strop, I also improvise with paper or cardboard quite often.  For sharpening the surface doesn't exactly need to be flat either like some may preach. 

Oddly enough I just shot a video doing a knife on a stone that was nowhere near flat, just to prove the point that you don't need a wet stone that is honed flat to sharpen a knife.  If I would have seen this post prior I would have shown some other improvised methods, but I am sure youtube has some vids. 

Growing up, I worked in a family shop and we always needed a small knife on us for various reasons and mostly that was for opening or breaking down cardboard boxes as well as other tasks.  We has a sharpening stone that looked very much like that one with the middle hollowed out.  Didn't know any better until I was looking harder into kitchen knives and sharpening those where they have those other stones to grind the non-level stones. 

I've also seen people use the coffee cup method as well.  Interesting what people learn and utilize. 

SNIP

That's pretty cool. 

Was watching the show Alone this past week and this chick was using her axe to split some smaller pieces of wood and she wasn't paying attention and she let the axe lightly drop on her hand.  Didn't seem like it hit that hard, but it must have been pretty darn sharp since it cut her open pretty good.   

Was sharpening some knives this past weekend.  Had some knives that needed sharpening and also tested some sharpening items/techniques on older blades. 

Overall, I can get knives decently sharp with the pocket sharpening sticks and gadgets, but I can't get them as sharp as how I can with whetstones, particularly like my kitchen knives.  Maybe some has to do with the steel many of the pocket knives were older/cheaper stainless steel.  Need to practice with the ceramic honing rod that I have.  I used to be able to hone my kitchen knives to a pretty sharp edge with that, but I haven't used it in years and it didn't seem to help with my pocket knives.  I ended up having to go back to the fine whetstone. 

Nowadays, I don't really use my pocket knives that much.  As I mentioned before, they are more glorified box cutters.  Even then, I have a utility knife at home that I have handy for that, but many times I find myself "forcing" myself to use my pocket knives just so that they get some use.  I mean, my pocket knives will get some light use here and there, but not much really.  Zip ties, packing tape, and the occasional bottle opener.   

Does some one run a class on Knife sharpening ?

I know theirs youtube videos, but seeing it up close and personal is easier for me to learn,

Was sharpening some knives this past weekend.  Had some knives that needed sharpening and also tested some sharpening items/techniques on older blades. 

Overall, I can get knives decently sharp with the pocket sharpening sticks and gadgets, but I can't get them as sharp as how I can with whetstones, particularly like my kitchen knives.  Maybe some has to do with the steel many of the pocket knives were older/cheaper stainless steel.  Need to practice with the ceramic honing rod that I have.  I used to be able to hone my kitchen knives to a pretty sharp edge with that, but I haven't used it in years and it didn't seem to help with my pocket knives.  I ended up having to go back to the fine whetstone. 

Nowadays, I don't really use my pocket knives that much.  As I mentioned before, they are more glorified box cutters.  Even then, I have a utility knife at home that I have handy for that, but many times I find myself "forcing" myself to use my pocket knives just so that they get some use.  I mean, my pocket knives will get some light use here and there, but not much really.  Zip ties, packing tape, and the occasional bottle opener.   

Honing with a honing steel doesn't exactly sharpen the knife blade, it sort of pushes the small dings at the edge back into place.

On my spyderco sharpener it has to sets of angles. 40 degrees for normal edge maintenance and 30 degrees for the back bevels. What happens over time is that the edge can become too obtuse and then you use a fixed angle sharpener, or even sharpen by hand at the wrong angle you aren't hitting the edge but a tiny bit above the edge. I have made that mistake where I make pass after pass and it wont sharpen.

One trick is to darken the edge of the knife with a sharpie. Then when you make passes on the stone or whatever, you can see where the blade is making contact and whether you are hitting the edge well.

I have my belt sander which will bring a messy edge back real well but doesn't make a super fine edge like a good set of waterstones can do. I have been wanting a nice set of water stones for a while. They work well for large kitchen knives, lots of real-estate so you can make a good pass and keep the angle steady.

Honing with a honing steel doesn't exactly sharpen the knife blade, it sort of pushes the small dings at the edge back into place.

On my spyderco sharpener it has to sets of angles. 40 degrees for normal edge maintenance and 30 degrees for the back bevels. What happens over time is that the edge can become too obtuse and then you use a fixed angle sharpener, or even sharpen by hand at the wrong angle you aren't hitting the edge but a tiny bit above the edge. I have made that mistake where I make pass after pass and it wont sharpen.

One trick is to darken the edge of the knife with a sharpie. Then when you make passes on the stone or whatever, you can see where the blade is making contact and whether you are hitting the edge well.

I have my belt sander which will bring a messy edge back real well but doesn't make a super fine edge like a good set of waterstones can do. I have been wanting a nice set of water stones for a while. They work well for large kitchen knives, lots of real-estate so you can make a good pass and keep the angle steady.

Yeah, I am aware of the purpose/use of the steel.  I actually lost mine when I moved back to HI.  I have a ceramic honing rod, which I use for the same purpose as the steel though.  I bought this ceramic rod almost 20 years ago now and IIRC, it has some light abrasiveness to it.

With the exception of serrated edges, concave sections of blades are a challenge for me when sharpening with whetstones.  I have tried to stay away from buying blades with concave sections anymore, but I have a few.  I have a couple of buddies that have the Wicked Edge sharpening system and that makes sharpening of concave sections pretty easy, but that system is pretty pricey. 

With the exception of serrated edges, concave sections of blades are a challenge for me when sharpening with whetstones.  I have tried to stay away from buying blades with concave sections anymore, but I have a few.  I have a couple of buddies that have the Wicked Edge sharpening system and that makes sharpening of concave sections pretty easy, but that system is pretty pricey.

You could buy a round stone. Or you could go real budget and just find a skinny cylindrical object like a dowel or pipe piece and wrap sandpaper around it.

The ceramic one is probably a very fine abrasive and can do more than just hone. Sometimes they need to be scrubbed to get the particles out of the pores and that sort of rejuvenates them.

Forgot about the Falkniven DC4 that I have..drrr. Its a great, pretty lightweight and versatile stone. Highly recommended. One side is diamond, the other stone.

The only blades I have in my emergency backpack is a Mora knife and multiplier tool.  For sharpening I have a fishing combo hook and knife sharpener which is real light and compact.