I use several different designs for my writing in general and a more specific design for my novel which has a picture of a wine bottle and glass on it. The computer printed cards work well if you're going to a lot of different events. At the recent L.A. Times Book Festival, I substituted my phone number with the booth location number so when I handed out the cards on site, people could find the location of where to purchase my book. At my recent booksigning event, I replaced the booth location, with the Barnes and Noble's store address and phone number. All these changes were free.
Free ink is always the best.
If you have children, they are an excellent source of free press and promotion. City events in the park, street fairs, parades, anywhere there are crowds, take the kiddies. PTA events, school and extracurricular activities all are excellent places to promote your book. Of course this won't work if your children are pre-teen or teenagers. Unless you're book is cool.