If you deleted them from an Exchange mailbox you can
recover them from the Tools menu or ask your
administrator for a restore.
If you deleted them from a pst you can try to restore
your items by following a guide
which is on my site
www.sparnaaij.net in the How To
section.
When you haven't closed and opened Outlook in between
you'll have the best results. If you have restarted
Outlook in between the results are unpredictable.
If the items aren't restored at the first attempt you
could try again and delete more or other characters at
the beginning but I don't think it will work.
Also the program SecureClean from
http://www.whitecanyon.com is said being able to do this
(haven't tested it myself yet.
The best way is to install the pst back-up add-in from
Microsoft or download my script from the download
section; preventing is better the curing.
Hope this helps!