Here I found a diagram of the lines through a curve, the yellow path being the ideal one:

This comes from a blog write up about cornering technique (http://realanalytics.wordpress.com/2...ring-analysis/)
You can see this in action by watching Fabian Cancellara in a descent; it's a clip I have linked before... you can really see how he changes his line in anticipation of each curve, how he leans into it, etc:
http://www.youtube.com/watch?v=RxXqQqAc2pA