You can create a file setenv.sh under tomcat/bin directory
The content of my setenv.sh is as follows:
JAVA_HOME="/usr/java/latest"
JAVA_OPTS="-server -XX:NewSize=123m -XX:MaxNewSize=123m -XX:SurvivorRatio=8 -Xss256k -Xms495m -Xmx495m -Djava.net.preferIPv4Stack=true -Djava.awt.headless=true"
You can modify the other memory size options according to your server's capacity.
Reference:
http://www.andowson.com/posts/list/556.page