I often cite to my teams (Canadian reference here) when Dan Carter scored two easy trys against Canada last year by simply shifting out into the gap.  He was then able to cause indecision among the two defenders who had to, because they were ball-watching and not focusing on their man, sort who was going to pick him up as well as the man outside Carter.  Needless to say, by then it's too late when facing DC, and he was through and gone over for five.

In terms of correcting/preparing for that, I'd be happy to hear what others have, but this is what we do:

Semi-opposed defensive organisation drill.  Most of us have this one, where the coach nominates areas for the 'breakdown' to occur and for the team to form up on, post/x/y (whatever you call it).  If you have enough to oppose them, then they can act as the attacking group, and (for me) post and guard/x are the only ones who are allowed to 'ball watch'.  The rest have to be looking forward, with reasonable quick side glances, nominate the person they're responsible for and keep them inside shoulder in-line.  Someone from the 'attack' group plays the ball and the guard/x calls 'up'.  In my defensive group, the SH is always a 'safety/sweeper*' who sits behind the ruck/maul and organises the post/x/y defence, AND calls ball out.  That communication allows everyone else to focus on their job and not have to watch for the ball coming out. 

*i.e. 'sweeps' across the backline as the ball moves out, picking up any short kicks or breakouts, but mostly there to organise and pounce on 'winnable' balls at subsequent breakdowns.

If you don't have enough to run semi-opposed, then maybe the coach could stand out where the first receiver might be and hold up so many fingers (something visual) for the defenders out wide to shout, PROVING, they're keeping an eye on what's in front of them and and not just the contact area.

The full back should also be playing a role here in organising the defence out wide.  One of the best analogies I've heard on the subject is to get the SH and FB to think of themselves as 'traffic cops' - the SH handles the congestion in the 'city' (i.e. tackle contest) and the FB way out in the wide open 'country' (i.e. in open space).