This is mostly an educated guess based on experience building practical props. I haven't been able to find any interviews or articles discussing the practicality of wearing the shield yet.
As far as the movie goes, I think the only method that makes any sense is a magnet. In the comic version, the straps of the shield are often visible over Cap's shoulders, indicating that an arm goes through each strap. In the movie, though, he just slaps that thing on his back and keeps going. It's obviously separate from the straps seen on the uniform shoulders, which is the harness you described.
If the metal bracket on his back had a magnet strong enough to hold the shield in place, it would snap on fairly effortlessly. The problem would be getting it to snap in place the same time every time. Of course, that's where the movie magic comes in - we don't see the numerous takes where Chris Evans went to slap that shield on his back and it clasped in place crooked or went flying off to hit a crew-member.
I would also assume that they have two versions of the shield, with one being a bit flatter than the "hero" version which is pretty curved. The curved version we see in several close-ups would be difficult to attach via magnet, since the center would be the furthest point away from the magnet. A flatter version with only a moderate curve (or a non-concave center and curved edges) would snap into place much easier, and would be used for the separate shots where he puts the shield on his back.
If you're interested in further reading, here's an RPF discussion where the topic is looked at from a more practical angle:
UPDATE: So far I've only been able to find one production photo that shows the back of the suit, and this is a stuntman so this version of the shield harness may be for looks only. If it IS functional, it supports either the magnet theory (probably located in the flat center section) or the slot/notch theory discussed in the forum link above.