Following up on Trim's suggestion, rather than blank card, something with a distinct picture or pattern might be more suitable. Autofocus works by sharpening up differences in contrast in whatever the camera's seeing. You could try auto-focussing using a suitable object, but then release the shutter button without taking a picture. Next, remove whatever you've focussing on, then press the shutter button fully and quickly. This way, the camera might (repeat, might), not have time to change focus before you've got your image.