Practice speaking fluently. Can you read out loud fluently? Then do it as much as you can. Are you expecting certain questions at the interview, or expecting to say certain statements? Then practice them in front of a mirror. Also, and this goes for even fluent folks, study up on the company, the job, the interviewers so you feel more prepared and confident when you go in. And get in a nice run or workout immediately prior to the interview, which usually helps boost your confidence.