I feel the same way. Practice makes perfect.

I had one interview. I spent 2 nights, (only a hour though) to go through the common questiona and self introduction. Even though, the actually question I got was different from what I prepared for. But I did quite well. When they asked weakness, I said communication skills. they were you are great.

But when I went for the second round, I didn't prepare at all. this time, they started to notice my english wasn;t that great. :P

so my point is practice helps you to warm up and build your confidence.