let me start
email :
don't send anything that you don't want to find on the newspapers
the next day. if you are sending dirty jokes or porno files through
office email, your boss might come to know about it. don't send
your personal info to someone you know only online. how would you
know he is really who he say he is? encrypt whatever you send if
they are potentially embarrasing. winzip with password is minimal
for your naked photos
surfing:
always clear cache and history when you finish, or your sis might
accidentally find a link to a porno site when she share your pc.
check your bookmarks regularly for things that were added without
you knowing. some sites do that. make sure you log off when you
finish, or someone might accidentally access your web mail account
or bank account.
general:
clear your recent file list and temp directories before shutting
down. don't keep your credit card numbers in plain on your pc, who
knows who is secretly monitoring your hard disk via a worm or
shared folder, or peer-to-peer programs such as kazaa?