In your catalina.sh file (if you're using unix), you must place this at the top:
export JAVA_OPTS="-Xdebug -Xrunjdwp:transport=dt_socket,address=8000,server=y,suspend=n"
Note the line may contain other options, but the above must be there nonetheless.
Now stop and start tomcat.
Then go into Debug Configurations in Eclipse and add a new Remove Java Application. Enter the following details:
host: localhost post: 8000
You can now start this debug configuration. Your application will stop and eclipse will move to a breakpoint added in eclipse should you hit on.