See that the laptop is always used in a place where there is sufficient airflow, which will help to dissipate the temperature being built up within the laptop. If there is no air conditioning, use of fans is recommended. Always work on firm surfaces, like tables and avoid working with the lap top on your lap. See that the surface is all always dust free. Monitor the temperatures on your laptop through software that can easily be downloaded from the net. These would even have safe and danger zones indicated while showing the temperature on the laptop. You can even have a sensor or probe which can measure the temperature, though the absolute figures on this will not in any way be an indication of safe temperatures. For this you would have to monitor the working of your laptop and judge the conditions under which you notice temperature rises. And naturally once you have these guidelines avoid long periods of working on the laptop in conditions that you know are increasing the temperature.
Long usage of laptops while watching movies or even editing photographs need to be done in such areas where you are sure that the ambient temperature can be controlled. Alternatively switch off the laptop periodically and take a break while the laptop cools down. And in spite of all this if you do find the performance is still sluggish or not what it should be, maybe it is high time you got your laptop to an expert and get his opinion of your problems.