I heard some web crawlers give you negative points if same website is available from more than one domainname. No idea if that's true, nor if it's a good idea... but if you really want to not have the main site on the address, your best bet would be
Code:
# if you don't have this line in the main config file, uncomment:
# NameVirtualHost *:80
<VirtualHost *:80>
# no ServerName, the catch-all "default" that grabs liXX-XXX, http://your.ip.addr.here/, and such
# uncomment one of
# Redirect / http://realsitename.com/
# or
# DocumentRoot /path/to/blank/page/or/something
</VirtualHost>
<VirtualHost *:80>
ServerName realsitename.com
Redirect / http://www.realsitename.com/
</VirtualHost>
<VirtualHost *:80>
ServerName www.realsitename.com
DocumentRoot /path/to/site
</VirtualHost>
<VirtualHost *:80>
ServerName www.othersiteIhostifany.com
DocumentRoot /path/to/othersite
</VirtualHost>
<VirtualHost *:80>
ServerName othersiteIhostifany.com
Redirect / http://www.othersiteIhostifany.com/
</VirtualHost>
# so on
Of course, if you prefer, make the www. version redirect to the no-www instead.
PS. Sorry for the edit, I'm sleepy and made a mistake.
