uh, just don't get your hopes up is all I can think of. I guess I could say look confident, people can see that always, even more when your the complete opposite as most people here. Just pretend your going there to get denied, so expect to not get the job, you might not be so worried and afraid of getting turned down so you might be a little more confident, who knows.
If it is a job just stacking items or working in the back room or whatever, then there really is no reason to turn you down unless they already had a better person, but if its as a cashier then you might not get so lucky, although if you get lucky then it could help you with being social.