look presentable, shave, wear dark jeans, etc. When I applied to Safeway (a long time ago..) I was an LB and i went in looking super scruffy with my pre-pubescent raggedy beard, dark jeans, and a lacoste polo with the collar popped! Got hired
Costco isn't exactly a fancy place .. just don't be awkward, notice the interviewers' breasts but don't stare for too long (it's like looking into the sun, gotta get it just right or you go blind!), know something about the company, be prepared to ask a question when they ask you if you have any questions