Do whatever you have to do to allay the nervousness and anxiety you feel on interview day. Run ten miles, listen to relaxing music, whatever. Projecting a calm, confident demeanor will do much more for you than a dissertation on the cardiovascular system.
Practice solid, coherent answers to the basic questions - why you want to be a CRNA, what resources (family/finances/etc) you have to get through the program, what direct experiences have you had with the nurse anesthesia profession. Know these answers cold so that you do not spend minutes with vague, wandering replies and "um" and "uh"s.
Avoid "desperate student" syndrome. You know...that deer-in-the-headlights look that screams, "this is the only program I have applied to, and if I don't get in, I am screwed!". Formulate pertinent questions, which illustrate that you are knowledgeable about the strengths and drawbacks of their program. Do not sit there silently when they ask you if you have any questions for them! Be prepared to show them that you have done your research, and that they should want you as much as you want to get in.
Understand that you may be asked to explain any weaknesses in your application. Take the time to formulate honest, straightforward answers so that you are not put on the defensive or caught making frivolous excuses. If you made a D in intro biology, but you were taking 18 credits, and it was your freshman year, then say so. If you made a D because you partied and drank too much beer, then say that you entered college without the study skills to excel in the more rigorous classes, but that you redoubled your efforts and subsequently maintained a 3.8 GPA.
Resist the urge to suck-up or gush. No one likes a sycophant.
When it is all said and done, thank them earnestly for their time and for considering you as a future student. Salesmen and politicians know that your tag line is the last thing people remember about you. Make a professional exit.
Best of luck!