Just say 'no' to all in-match coaching. I'd like to see the players boxes moved either to the middle of the far side, so they couldn't even give advice on line calls and too far to give any verbal coaching - or better yet, let them watch from TV in the players lounge.
This is nice in theory but it's pretty much impossible to implement. Coaching existed all the way back in the 70's and flourishes in today's tennis.
Ok, so you ban coaches and staff to come to said player's matches. Big deal, they will just hire certain people to go incognito in the stands and give out "advice" to the players, informing the player beforehand where they should look/listen.
Every ump knows that the players gets some coaching in(looking at their box to challenge is the most obvious example) but they only give out warnings when it becomes too blatant for their liking. But still, blatant or not, it happens on a regular basis, far more than people here like to think.
I'd bet that in the next 10 years or so the ATP will go the way of the WTA and allow coaching. It already happens on a pretty big level and since it's almost impossible to truly stop, it's the next obvious step.
You should either enforce a rule or abolish it but the problem with the coaching rule is that it is vague and open to interpretation. In theory, ANY communication between the player and the box is coaching, even shouts of encouragement. Tell me of a box that will stay silent when their player is having a tight match. Not many I'd wager.