For me personally, when it comes to humanoid and creature modelling I can visualize and improvise as I go, even with very little references. This is because I see faces and body shapes everyday in the real world, but when it comes to hard surface modelling—let us use Iron Man as an example here—I run into problems (though I haven’t really given hard surfacing a try yet) and have a hard time visualizing. It takes skill (I think) to decode a blueprint, so for me, a front and side view aren’t enough! I believe the best way to improve in this area is to really get a hold of, for example, an actual Iron Man figure. The best references are the ones that you can touch, feel and can see from all angles.
I was at a toy store today looking for a Stormtropper but couldn’t find the head and ended up taking a few shots of something else.