Private Blog Networks are best used in a couple of different ways.
1. Direct linking – A good private blog network has no footprints. The domains are not connected in any way including the registrant info, nameservers, IP address, theme setup, content, etc.
Direct linking gives you the most bang for your buck by giving your main website the most linking power. Using the next strategy is safer but diminishes the power.
2. Tier 2 or Indirect linking – This method would be to build a good link such as a guest post, then use the blog network to link to the guest post, the second tier.
This is a very safe method as only your high quality guest post is linking to you, but in the end you only have 1 link pointing to your site. For most keywords, you’re going to need a lot more incoming links (IBL’s).
Either strategy can be effective, PrivateBlogNetwork.info usually recommends the first approach as it gives you the most value and the best rankings.