How can I get more miles out of my tank of desiel?

I have a Ford Focus 1.6LX TDCI which is a 05 plate. I have own the car since February 2006 with only 10 miles on the clock, up until March 2007 I was getting about 510 - 530 miles to the tank of desiel. Since March 2007 I have only managed to get about 460 - 475 to the tank of desiel. Car has been serviced once and a spring check carried out on it. I am able to get the same amount of fuel in the tank as per normal but not the same millage. Can any one tell me why this is??? or is this a standard feature on desiel eninges???