If you are making a lot of cards with the prick and stitch method it is well worth getting one of the purpose made pricking mats that will be sold by your favourite craft store. Pricking mats are usually made from compressed fibre, felt or polyurethane foam. The size will be around 9 x 6 inches (22 x 16 centimetres) and ½ inch (1 centimetre) thick.
However, if you are new to stitching cards then you may want to improvise before you spend your hard earned cash. I have heard of people using the back of a mouse mat. I think this would be too thin on its own to allow the pricking pin to penetrate far enough. You could put the mouse mat on top of a couple of magazines to give added depth whilst protecting your work surface.
Another possibility is a folded towel. It should be folded enough times to allow the pricking pin to penetrate at least ¼ inch (5 mm). Carpet off-cuts, the type of polystyrene used for packaging electronic equipment and foam rubber are some other possible ways of improvising a pricking mat.
Please let me know what you are using as a pricking mat by adding a comment to this post.