I've found that I struggle the most with the shoulder plates, and in cases like Njal's, the librarian terminator armour. The rest in't a struggle, and mainly to get the dimensions and proportions to match so that the marine looks like it should be the way it is.
those pauldrons are an awkward shape...i sometimes try and visualise a cube shape. (its very obvious in my drawings when i don't lol). so try drawing a cube in perspective where the shoulder pad should be, then draw the semi circle shapes inside that cube/oblong. it takes a little practice but you'll get the hang of it.
Ah, awesome! I've wondered about ways to do that... My Dark angels army is finally assembled, now for primer and paint 
cool, enjoy 