In every movie Iron Man has been in, there's a new way that the armour attaches itself to Tony. In the first one, lots and lots of robot arms fit the armour onto him. In the sequel, he has a suitcase that transforms itself around him (my personal favourite). In The Avengers, he has bracelets which the suit detects and homes in on, attaching itself to them and then transforming around Tony. And then, in the new movie coming it, it's as I described above.
Like I said, my favourite of all those is the suitcase armour. For those of you who don't know what that looks like, here's a clip:
Yes, I am aware that sounds completely ludicrous, but I know exactly how I'm going to do it. It's really not complicated. At all.