Problem with PermGen Space in Jboss

Hello everybody
I have a problem with JBoss, that is: I did deploy a project and after a
few hours of execution (without new deploy) I have "Java.lang.OutOfMemoryError: PermGen space".
Why garbgage collector does not succeed in deallocate memory? What may be the problem?

The question associated with NetBeans: do you think that it could be a good idea to set to null all the variables of all the methods of all
the classes of the project in a finally block? (in this case, in Eclipse, is there a tool that allow me to do this
automatically?).

Thank you everybody for your attention!

Don't spam the forum with the same question.