Every now and then I come across people wondering why their network of sites is having problems.  They think they built it correctly and yet something went wrong.  Here are the key issues I would pay attention to if I were to build a network of sites.

Domains - Use different registrars, on different dates with different whois information and register the domains for varied time lengths.  This will randomize it but it also does create problems since false whois information can cause you to lose a domain.  The alternative is to use the private registration services.  But that will look weird if you have 100 sites all using private domain registration. 

Hosting - Forget about different IP addresses.  You need to use different vendors.  It is easy to compare the IP ownership and see that it is all hosted at one company.

Templates - Do not use the same look and feel.  It is easy to find sites by searching for identical footers or copyright notices.  You need to pay attention to the small stuff and make sure it is all different.

Toolbars - Most toolbars today are tracking your internet activity.  If you have a toolbar installed and you regularly visit your secret network of sites, your repetitive traffic could show these sites are related.

Links - If you interlink these sites it is especially easy to uncover that.  You need to randomize the links.  Also if the only inbound links are from your network, it is easy to determine that as well.  You should add some “noise” to the network and add easy links from outside sources like blog links, reciprocal links, etc.  Also outbound links are another easy item to investigate.  Make sure to add “noise” to the outbounds links as well.  And I think it goes without saying to randomize the anchor text a bit.