Attending an interview, there are many things you have to prepare for. Academically, presentation wise and also the way you speak. Here are a few things that will help you with the way you speak in an interview.
Interviewers hate to hear the filling-in-the-pause words like umm, like, and, uhh. They can get on the nerves of decision makers apart from associating the words with indecisiveness. Instead, you can make a conscious choice not to use these words and replace them with more intelligent alternatives.
You can you, "for instance" instead of like which will also add to the quality of the speech. If you are completely lost for words, calm down and take a complete silent pause and think about what to say instead of just filling up the gaps. The pauses which seem to be very long are not very long in fact, you can observe if you record yourself.
Also, the interviewer will clearly know that you are the thinking-before-answering kind when you take an intelligible pause before you say a few lines.