Buy quality-brand briquettes. They burn hotter (and longer) than lesser-grade ones because the charcoal material is denser and thus more combustible. In addition, you can increase the heat that reaches the food by placing the food closer to the coals, using extra briquettes, and packing them more closely together.
You can also augment the heat by increasing the speed of the fresh air flow to the briquettes. (Open the air vents under the barbecue pan, if your unit has them). However, be aware that air flowing over the food will both cool the food and carry away some of the heat rising from the coals. (Try to shield the food from any passing breeze).
Another way to raise the temperature is to minimize the heat that escapes through the walls of the barbecuing pan. Buy a unit that has a pan made of thick-gauge metal, or insulated a thin-gauge pan by lining it with a layer of ashes or aluminum foil. The aluminum foil also reflects heat upward to the food.
** Asian Recipes