Above all else, be positive.
If you are currently employed, use this opportunity to focus on what kinds of things you’re looking for in your next job (not the things you hate about your current one). As you already know, pharmaceutical sales offers a highly competitive, cutting edge environment. If you dig science, say so. If it’s the marketing that gets you going, say that too. Finish by talking about how well you know you’ll do in the job that meets all of the demands you seek.
If you’re currently unemployed, be positive while telling why you left. No boss wants to hire someone who complains or makes excuses. Talk about how the unemployment has led to the opportunity to work as a pharmaceutical rep and then describe what you bring to the table (again).
If you find this site helpful, please send a link to your career services department so your fellow alumni know about PharmBoard too! I've already written the message for you. :)