This ends up being camera specific, so there isn't one easy solution. You have to experiment.
Like Andaah said, the macro setting, if your camera has it, may help. Using a tripod, or some other way of holding the camera perfectly stationary should help a lot too, as shaky hands can play havoc on digital cameras. Also, if your camera has auto-focus, try switching around the background, lighting, and distance between the camera & model, as the auto-focus may 'like' one combination more than others.