^I love that everyone's picking up on that even when all I have to show you is his rough sketch. He's a slender, conniving monster rather than the tank that your character is - I think it's a nice contrast between the leaders of The Hive.
@Wolf: All of my sketches are done on an iPad with the SketchBook Pro app, but I smooth them out and color them on my computer with Photoshop CS5.
Early on in this project when I wanted to get it anatomically correct, I just sort of drew them while looking at reference images. Then I moved on to looking at different types of Xeno heads, and eventually I just knew the shapes and placements of the things you see. Sometimes I mix and match certain characteristics.
I start out by making the basic shape of the dome. It doesn't have to be perfect, it's just a guideline. If you noticed, the front of the Xeno's head is actually sort of bumpy rather than completely smooth (unless you're looking at the modern designs). You can look at it however you want to, but this small bump (the forehead) combined with the long banana-shaped curve of the rest of the head is shaped exactly like dick and balls.
One of the most defining characteristics of a Xeno's face is its large cheeks because they form that subtle yet incredibly creepy smile it always has. Also notice that the side of its mouth barely has that "ripped flesh" look to it - a lot of people put way too much emphasis in making the sides of its mouth all messed up. And it's fine, it's a good look, but I prefer the more human-looking mouth, which seems to have been retconned in newer material (the blu-ray discs of Alien and Aliens show the original Xenos with the ripped flesh mouths rather than the subtle smiles).