NHibernate version:2.0.1.4000

i retrieve object from DB using NHibernate then keep updating it in ASP.Net session through many postbacks (like wizard steps)
and final when i try to save it i got the following error

Illegal attempt to associate a collection with 2 open session
how can i work around this error ?

something like this:

object myClass = blahblah.load/select/whatever

object myTransientClass = new object()

(marshall values from myclass -> mytransientclass)

store mytransientclass in session/cache/etc

Upon confirmation, grab myClass from the persistent store, update with the transient values, then flush the session?