Does applying for a credit card then cancelling it out hurt your credit?

I cannot resist the urge to get an extra $20 bucks off a purchase by applying for a credit card. Then I pay it off and cancel the card. Does this hurt my credit rating? I bought a wet dry vac today at Sears and managed to get $25 bucks off by applying for a Sears card. When I do this I always pay the balance off within 30 days. Should I quit doing this? Should I cancel the card everytime?