If you are using a "kettle" style cooker, such as a Weber, you can control the heat by allowing the fire to settle for about five minutes after the coals are ready. Spread the coals out evenly, place the lid on the cooker and go do something else. Come back in five minutes. If the fire is still too hot, close the bottom vents about halfway and check again in another five minutes. If it's really windy you may need to close the bottom vents nearly all the way. You could always mist the fire with a spray bottle as a last resort.
If the fire is too cool, they are either not lit properly or they have begun to burn out. In extended cooking, I find that a small kettle cooker, like a Smokey Joe, is handy for lighting more charcoal in a charcoal chimney. That way you can add lit charcoal to the big cooker and heat it right up.
If you are using a grill without a lid, you will amost have to get a spray bottle. Best of luck.