I use
Olfa AK-3 knife which I find perfect for work in paper. I almost never buy new blades though - instead I sharpen them again and again, and even if the pointy end breaks off I grind the back of the blade with my dremel to get the sharp point, then sharpen the blade again.
How to sharpen the blade? In paper work the blade rarely is dented - so sharpening it is more like smoothing than grinding. Using VERY fine sandpaper I move the blade as if I was cleaning it's side on the sandpaper, in direction towards the blunt edge of the knife - like on the linked part of this
youtube movie.
I covered one side of my steel ruler with fine sandpaper, using double-sided adhesive tape and sharpen the blade on it, with the ruler lying close to the edge of the table.
When to stop sharpening? You can see that a knife is blunt when looking at it's edge directed towards you, in light of a lamp you can see it's edge shining. Well honed edge is all but invisible.
Additionally the sandpaper stops my ruler from slipping when I use it for guiding the knife when cutting paper - allowing for a more precise and easier work.
I use fresh knives very seldom - for extremely demanding tasks, and to switch to a new knife when the old has worn out.
It goes without saying that it's very positive for my budget