One thing that has not been mentioned (as far as I can see) is that the pst file may shrink if "compacted". This removes space used by deleted messages from the file. If you have not deleted many messages then this will not help much. Copying messages into a new pst file (which I think has been mentioned) will of course also achieve this (if that is indeed such a thing - it is beyond my experience).
If short of email space focusing on messages with large attachments can provide big gains with small effort. Outlook (some versions?) can display messages sorted in order by size. I have in the past been able to easily spot duplicate messages with large attachments that have been forwarded back and forth creating multiple copies of the attachment. Delete the desired messages AND then compact the pst.