Learn about the fees that a store pays, including the percentages, the machine/software cost, the phone line cost, the additional costs associated with CC fraud, etc. that gets passed onto the consumer in the forms of higher pricing, etc. Do you ever use cash and ask for cash discounts?
Also the things listed in another post here (CC companies use as a tool that spending with plastic consumers buy additionally more then they would with cash).
If your buying more because of plastic (no matter if you pay it off every month), then that money back is really just money that you may not have spent in the first place. (free loan?)