I have used a postcard stamp similar to what SingerSewer describes except that it wasn't the full size of the postcard -- just had the "postcard" word in that vintage style font. It looks nice. Then I have drawn the center line on.
I enjoy making those postcards, but my problem has been getting a standard postage price from postal clerks for mailing. They don't seem to agree, and I don't want my recipients to have to pay postage due.