As a School social worker I do mock interviews with 5th graders preparing for the MS interviews. One thing we stress is the importance of 1st impression - eye contact, manners, attire, etc. Also - helping the students to coherently express their future goals (not to lie, but prioritize - if you're applying to a selective MS, perhaps your goal of becoming a "famous basketball player" should not be the 1st one you mention); also - realistic portrayal of their strengths and weaknesses, both personal and academic. and...all modesty aside, how to present yourself in a positive light (many kids have a hard time with this one)
hope this is helpful