Lots of people have been asking this question lately..My recommendation would be to at least do the following: -Mask/Change Whois Details on all domains -Host on different IPs -Use different themes and admin usernames -Don't link them all together. Link them all to your money site, but strategically link between your PBN sites or don't do it at all. If you want to build authority on your PBN sites then consider doing Web 2.0, Article submissions, social bookmarking..etc.
Another important thing is to use Unique content! Don't even use spun content! Also be sure to use About me pages, DIFFERENT (TOTALLY DIFFERENT!) domains for your web2.0's, PHOTOs and Youtube VIDEOS!
depends on how they are and what ur aim with the site is(short term or long term). Things to look for: no footprints, quality domains, good content, good PA/DA.
We can't say "No" yes its still working but its depend on you how u ll get maximum advantage for your site promotion.
In case of using web 2.0 sites like tumblr or blogger how to not leave any footprints? I don't have much money right now so I'd rather use to buy good content instead of good domains so I would do it all on web 2.0 websites.