Drink plenty of water. Ask for water even if you're also having wine, beer, or a cocktail. By drinking lots of water, you'll end up drinking less of those other beverages and consuming fewer calories. Chances are you'll eat less, too. Have it your way.
Chefs care a lot about making customers happy.
And good chefs can do plenty to make a meal lighter and still flavorful. They can grill a portobello mushroom or sea bass instead of frying them, for instance. They can saut zucchini, eggplants, or onions instead of battering and deep-frying them. They can take the couscous from one dish and pair it with the sauteed swiss chard or string beans from another to put together a savory light meal. So be assertive and ask for exactly what you want.