just tell a little bit about yourself, why you want to work, talk up your work ethic.  throw in there that your family has been shopping there and you have seen what a fine store they run and you want to contribute to the reputation of the store..
and then depending on the job just swap out the last part(telling why you want to work at the certain store)   it seriously works, i have held the same 2 summer/ fall jobs for 3 and 4 years respectively..
i turned down the grocery store, but i was still offered the job on the spot, non of this we will get in touch BS..