There is really no reason to clean up your Sent Items in GMail (unless you have managed to run out of space) but here is how you can do so if like me you just want to tidy things up.

How to remove all sent items which are not labeled:

1. label all sent items with a new label such as TEMP
2. view e-mail for another label and add "label:TEMP" to your search query
3. remove label TEMP from each item in these search results
4. repeat steps two and three for each label other than TEMP

All items with label TEMP should now have no other label indicating they are safe to remove as they are not part of a labeled/archived conversation.

In order not to loose any archived items you wish to keep you must ensure everything you wish to keep has a label. This should be
done as you archive items rather than all at once when you are trying to cleanup. I created a "MyArchive" label for this purpose.