As soon as you hire, call or write the applicants who didn't make the cut and tell them you'll keep their applications on file. That way, if the person you hired isn't the best--or is so good that business doubles--you won't have to start from in hiring your second employee.
For each applicant you interviewed, create a file including your interview notes, the resume and the employment application. For the person you hire, that file will become the basis for his or her personnel file. Federal law requires that a job application be kept at least three years after a person is hired.