[PICARDIE]

Here is the tool I use. Available in the "kitchen tools" department of any supermarket. It is intended for sharpening knives, it too works well for sharpening the X-Acto and other similar blades. The grindstone is shaped as "V" wheel, so a few back and forth movements of the blade in the "V" are enough to sharpen it.
(Sorry for the poor quality of the images)