This is a good question. There are so many different types of promotions for customers. Some of them are very clever. I will mention three of the most effective.
First, many stores have promotions that are structured in such a way that you save more when you spend more. In other words, if you spend more money, there is a greater percentage off the total purchase. The aim is to make customers spend more, in order to get a greater percentage off of the final price.
Second, some stores give coupons for every hundred dollars or so a person spends. The idea behind this is to bring people back into the store to redeem the coupons. What they want is repeat customers.
Finally, the newest thing is something called groupons. A vendor offers an insanely low price only if they a certain number of people buy that product. The idea here is trying to sell something in bulk.