You have different options, please take a look at the persistence configurations inside the official documentation to understand how to work with it.
You need to think how to maintain your session live in a global scope or how to restore it before interacting with a human task. Doing that you will be sure that your session will be notified by the human task server when the task is completed.
You can also take a look at the examples provided here:
https://github.com/Salaboy/Drools_jBPM5-Training-Examples
and here
https://github.com/esteban-aliverti/JBPM-Samples
Both repositories are part of a community training about jBPM that we are creating and improving here:
http://salaboy.wordpress.com/2011/01/24/announcing-jbpm5-community-training/
Cheers