If you've done research you should be prepared to provide a summary of that. Your summary shouldn't be too long and should be understandable by someone who has no expertise in whatever field you worked in. Be prepared to discuss your motivation toward the field and any experiences you've had in it. Come up with some reasonable career expectations and know what field of research you want to work in and why. Come up with some specific reasons why you are interested in their particular program. Be ready to address any weaknesses on your application. Look up standard interview questions and be prepared to answer those, as well. Finally, come up with some good questions for your interviewer and try to think of more questions as they tell you about the program.