Rubber or plastic bumpers in the four corners of laptop bottoms stabilise the computer and also help to keep the laptop cool by adding additional airflow. After a year or two many of these bumpers are lost and it can be difficult to find the specific size for your laptop. Consider using Sugru to make replacement buttons.
Andy on the official Sugru weblog details how well this solution worked for his MacBook Pro. He fashioned his replacement bumpers to hold the laptop a bit higher than normal for even more airflow.