I was wondering the same thing. Im thinking about attaching magnets to a zentai suit underneath and putting magnets on the inside of my iron man pieces and line them up as i put them on so they snap together. There is probably a simpler way though.
If it's a foam build, Stealth did a few great tutorials on the subject. You could probably apply the same method to any other material as well. Indiefilmgeek also goes into detail with how he put together a suit for his son. Either way, they're both great sources of info.