The main part of the helmet is just three drawn curves and a couple of image maps.
This video outlines the process:
Helmet Video
Here are the drawn curves:
The helmet was improved using a transparency image to hide the bottom of the model (leaving the curved shape of the ear guard) and to cut out the air holes. I used Project Dogwaffle to make a TGA with an alpha channel for this.
Then a curvy map was painted to add the small surface shapes and rim.
The face guard is just several drawn lines. The plastic clips on the guard are extruded images (Curvy 2.0 only).
The best bit about modelling using images rather than meshes is ease of modification. Once I had made the helmet I realised I needed to move the ear guard forward a couple of inches. All I needed to do was tweak my image maps (happily saved as easy to change vector art layers in my paint program) and it was fixed.