I'm sure others have their own way, but I can tell you what I did to have my sent emails also appear in my sent items. Note that I don't use IMAP, I'm not on BES and I use Outlook.
1) Create an email for this task. (I have a domain, so I created an email account
)
2) Log into your BIS and enter this new email address in the Auto-BCC field
3) In Outlook, set up a rule to have emails sent to
, go directly to the Sent Items folder
Like I said, I'm not the most tech-savvy person there is, so I'm sure there might be a better way to do this, but this is what I do and it works for me.
Hope it helps.