Somebody who has never tried this out will not be able to truly understand how difficult it is. The nature of fish meat is that when it gets cooked it starts to flake, and inevitably with a barbecue, the fish will start to get stuck to the grill.
The temptation is there to try and play around with the fish; continuously teasing it from the grill to stop it from sticking, but this temptation must be resisted.
The best way to barbecue fish is simply to wait until it is done. Throughout the cooking process, you should be trying to lift it of the grill; if there is any difficulty with it lifting off it, like there is a bit which has cooked onto the grill, then you should leave it, and eventually it will be cooked enough that it can just lift straight off the grill.
Another important part of the process for perfect barbecuing of fish is to try and cook it at a low temperature, as this will stop it from getting burnt before it has a chance to firm up.