If you buy an item that later is placed on a rebate, you can bring your membership card back to the store (receipt not necessary) and get cash or credit refund for the difference between current sale price and what you paid. I believe there is a time limit on this of 14 days, but I have definitely taken advantage of this policy.
If you see an item return to a club that was available previously and put on a rebate, wait to buy it because chances are quite high that it will be placed back on a rebate a couple of weeks after arrival. This happens with wine as well but is not quite as reliable since alcohol sales follow different rules (no returns on alcohol in Georgia, for example).
The coupon book is the obvious one but I have noticed that it cycles through the discounts at fairly predictable intervals so I stock up when things are marked down and rarely buy things like Starbucks, Tuna, sardines, crackers, without a mark down. You can also get a price adjustment within 30 days of purchase if you see a price drop in the next book or it goes death star.