I have seen a link science site that had an algorithm, where 6 sites trade links and link to each other one way at a time. it is confusing unless you can see the drawing, but i think its got to be virtually undetectible by search engines.
1) Directories - I use directory submitter to automate this (sw you buy) or you can use the link i provided to manually do it. 2) Link Ad Coop via Receivelinks - My favorite along with Link Vault. Receivelinks allows you to limit to only relevant links which makes it a winner to me. Put 1 site in the network and use that to point to some of your more important site(s). I avoid DP Coop. 3) Article Post Robot - I just started using this after giving up on online submission services and LOVE IT. Works great, although I hate the delay put in.