I've added a zip file with an STL of the Ironman helmet pieces for others to use. Please credit the Replica Prop Forum and I, if you use!
Step 1: Modeling
The first step, while the poly count was low, was to extrude the model inward to give the helmet thickness. Thickness is important, it provides stability to the final product which can be brittle when it is printed too thin. Because I planned on wearing this, (and probably walking into walls/doors/people) I wanted it to be thick enough the handle a bit of abuse. The final helmet varies in thickness because while the outside has grooves and ridges, I wanted the inside to be smooth. That way, the sides and bottoms of the grooves would have extra material around then to prevent fractures. The overall helmet averages between .125" and .25" thick.
When I was done adjusting the silhouette of the low poly helmet, I subdivided the model to provide a smoother finished surface. As you'll see later, I could have gone even further with this step.