It would be really helpful to see the file.
Without seeing it, my best advice is to "fill" it in smaller portions, or try using a Boundary Surface.
With both tools, you can add in additional guide curves to help constrain the new surface to what you want.
You can also control how the surface interacts with the surrounding faces, through contact, tangent, and curvature continuous options.
I've never done the Audi R8 tutorial, but are 3D sketches used like this to generate the boundaries of the panels?
I believe you'd have better results by sketching the curve from two directions (each a separate sketch), and then using a projected curve to "merge" the two profiles into a single 3D curve.
If you turn on "show curvature" for the 3D splines, you'll see that they are filled with ripples. If the boundary is filled with ripples, the resulting surface will be as well.
I'd also suggest the use of the Boundary Surface tool for this shape.
Fill might work, but I general use Fill for surfaces with three sides. Boundary tends to do a better job when there are four or more sides.
Have you downloaded and reviewed existing r* models to see what method(s) were used?