I have recently came across the need to have a "Preview" before posting a Topic, Reply, Private message in In-Bulletin.
Let's say we have 2 buttons Submit and Preview - it will reload the page with Post Preview (all BBCode and Smiles processed as like we have posted already).
Then below we have Textarea with actual post so user can modify things right there.
Let me know if there are improvement you think can be added? Perhaps Ajax processing or something else?!