From License to Grill, by Chris Schlesinger and John Willoughby.
THE NINE-STEP PATH TO GRILLED WHOLE FISH NIRVANA
Choose the right candidate and make sure it's really fresh. If you're grilling one whole fish per person (which is the best plan), you should look for fish that weighs between 1-1/4 and 2 pounds. Any larger than that, and you'll have trouble flipping them.
Start with a big hot fire and let it die down, catching it just as it turns from medium-hot to medium.
Make sure your grill grid is hot and clean. The best way to do this is to put the grid over the coals just after you light the fire, scrub it well with your wire brush, and then leave it on as the fire heats up and dies down.
Oil the fish very lightly before you grill it, and make several horizontal slashes about 1" deep in the sides of the fish -- these will help it cook more evenly and also make it easier to tell when the fish is cooked properly.
Lay the fish over the fire and don't fool with it for 4 or 5 minutes. You want a good sear to form between the fish and the grill, which makes it easier to turn when the time comes.
After 4 or 5 minutes, use your tongs to sort of worry the fish free of the grill surface, Don't turn it at this point, just clear it from the grill.
Continue to cook on that first side as long as possible, until it's just in danger of getting overdone on that side. At that point, roll it over gently don't flip it, just roll it, as you would a sleeping bed partner. This is the only time you'll be turning the fish.
If the fish should rip apart when you are moving it, it's time to bail. Get a spatula, lift the pieces off the grill and onto a baking sheet, and finish it in a 350 degree oven, It will still taste great, but the presentation is going to suffer.
To check for doneness, probe around in the slits you made in the side of the fish. The flesh should be almost completely opaque with just a hint of translucence. Remove from the grill and eat immediately: you have arrived.