You can also get something called a "mirror stamp". It's just a plain, flat piece of rubber that you stamp your image onto and then use it to stamp on your paper. Works pretty much the same as the acetate technique in the tutorial in the resources section but the ink transfers just a little differently. If I do if the acetate way, I always seem to end up with my ink squishing out and distorting the image slightly (which is fine if you're actually making a reflection effect since water etc would do that but not always so good for other things). I did the bears on this card with a mirror stamp:
Bear Hugs. The original bear stamp faces left so the one facing right is the one done with the mirror stamp.