Put your computer in the coolest place available, preferrably somewhere with air conditioning, and be sure there is space around the box for sufficient airflow. Open the box and remove any dust that has accumulated, particularly on the heat sink and fan. The next step would be to remove the heat sink from the cpu (when the computer is off!), clean off all the black thermal pad junk from the heat sink and the cpu using isopropyl alcohol and a clean cloth, then apply some arctic silver thermal paste to the cpu and reattach the heatsink and fan. This would improve heat transfer.
Adding fans, new heatsink, etc are other options.