If I'm outdoors without a sharpener, can I just use a relatively flat rock, or are there better ideas.

Sandstone will do nicely, otherwise granite etc. If it is smooth and flat even better. I have used a brick and glass to sharpen a blade in an emergency.

If one is wearing a leather belt, knife in a leather sheath,wallet or leather boots, the leather itself will help keep a knife sharp. I usually keep a small (1" x 3") piece of leather in my survival kit for this purpose. After I use my knife for any cutting purpose I always give the knife a few passes over my piece of leather. Doing this always insures it is razor-sharp.

Good idea! Using leather. Do you use it like a strop with the blade facing away from the stroking motion? Also would rubbing fine dirt into the leather help or be harmful to sharpening the blade.

If you use leather, it has to be used like a strop or you´ll slice the leather. Rubbing in dirt tends round off the edge. You should also keep in mind that leather only helps as long as the edge is still fairly sharp. If you didn´t bring a sharpener, fine grained gravel or sandstone will help when the edge is blunt.