All the sharpening systems mentioned work....but technique is when gets the job done right. I was taught with an oil stone many years ago. Don't use it much any more. I use a sharpener that is diamond dust impregnated on a steel. Bought it from Calelas many years ago. Have a real small one about the size of a thermometer I use on fish hooks. The downside is it will wear out a knife much faster.
The one I like best was the cheapest and does a great job to put an edge back on a knife when you are butchering or dressing out an animal. Can't remember the name, but it is a small yellow plastic thing that has a course and fine notch on opposite sides. Bought it at WalMart for a couple of bucks. I keep one in my pocket when I am in the field, works great to dress up a knife that gets a little dull while you are using it. It only takes seconds to put the edge back in fine shape. Quick & easy