Definately don't lose hope until the end. My friend last year was looking at a shutout, and then suddenly in March he got emailed from one of his top schools b/c the first interview went terrible. So the prof called my friend and he ended up getting in. Point is, there's always a chance of second-round interviews. The "hot shots" will sweep up all the first rounds of interviews. But, they can only go to one school!
In the worse case you take a year off, gain some lab experience, retake GREs, just push hard as you can. This is my second time around, and the time I took off in between made a big difference not only in making me more competitve, but helping get a grasp on what I'm most interseted in studying the next many years of my life. This is SUPER important - I know some people who got into PhDs right out of undergrad and burned out quickly because they never took time to really think deeply about what they wanted to do. Finally, just remember there's a lot of luck invovled!