Different Corner, Different Line
The racing line becomes much more complex if you think about that almost every corner is not the picture perfect line as illustrated above. You have to deal with a lot of factors, such as speed, corner layout, surfaces, steepness, bumpiness, obstacles, etc. - those are all common issues in the average corner.
Then theres more to take into account: Not all corners are followed by a straightaway. When a corner is followed by one or more corners you still have to try to maintain the best racing line for the first corner. But you also need to set up the car to tackle the best racing line for the next corner. So by compromising your line and speed for the first corner, you can get faster through the next corner.
This way you increase exit speed for the straightaway to a maximum!
Try and experiment with different lines, different approaches and different braking points. Usually there is only one best line through each corner. Try to find this line and practice it to clear corners even faster.
THE 21KM LONG NÜRBURGRING NORDSCHLEIFE
For a good idea of the racing line go and take a look here at this guide for the ideal racing line (Adobe PDF file) for the Nürburgring Nordschleife circuit, developed by BMW Motorsports. The Nordschleife circuit in Germany is probably the most demanding racing circuit in the world, so its a good opportunity to actually see how complex a racing line can get.