Lots of people go on about footprints and how to avoid them, well there are oodles of things that the bad people (Google) can do to find your sites, which you may or may not be linking together in a sneaky fashion.
I have seen Data in Star Trek and I get the idea that patterns can always be found. In this case Google and Data are the same, trust no one, least of all them..
The footprint should not only be the concern of the Blackhat, it should be part of what the cleanest of clean SEO’s look into. Why is that, well Blackhats anticipate losing sites, but when your real site goes MIA it can be teary eyed times.
Also competitor people snooping around, you don’t want them seeing what works if you can help it or let them know that you may be behind some good raking.
I have seen grown men nearly cry (the big Jessies is what I am saying and sticking to it) when losing a lot.
Now this is not an exhaustive list, just the sort of thing I would be looking for in clients SEO programs or link networks etc etc before I get bored or have to take my dogs out for a poo. The dog is ticking!
1. Whois details. If you have daft enough to buy all your interlinked sites with the same whois info you have done a major faux pas. Sorry to say I would reckon that Google have access to all whois, with history too. If they say they don’t have access it, it is your duty to not believe them and assume they do. I would, if I were them sneakily buy this info, they have lots of cash and this is a cold war of sorts. Bad search results can make share prices drop! Vary NS, registrants, have legit whois info. You need real for real sites as you can get that letter from Nominet saying you ain’t real 
2. Loose lips sinks ships. If you tell no-one or not many people about your stuff you can’t get it outed or investigated. A friend of mine has some amazing rankings and pushes the boundaries. But loads of us know his sites so we can look at what he is doing and maybe recreate it. If he never told us who his client was we would not have mooched around in the SERPs to find his stuff. If you value your private cutting edge stuff, keep it that way. That means, not sneaking a link from a performing site for your own private stuff (cheeky!) or getting that not really needed foot link.
3. Same templates. Using the same designer or free templates might not be the best thing in the world. I like to adapt (ahem) other people templates to remove footprints of links home. Naughty I know, but hey no-one is going to die, it is not brain surgery is it? To mix it up fella, keep it random. I suggest using different designers as even when told they need to be different some designers flip things around with CSS and an image change but 99% of the template is it the same.
4. Coding footrints or onpage footprints. Ok obvious stuff like saying the same think on all your sites (simple example) can find your whole set of sites. Taking it a stage deeper and on a similar line to CSS faux pas below.
5. Same CMS. If you always use the same CMS, or blog or whatever, well you don’t need me to tell you more do you?
6. URL structures. Always using the same structure, be it folders or html, htm, underscores etc will be a big obvious no no. I have seen sites in sectors I watch and I know who owns them before I even check it out thoroughly.
7. Same webserver techs. Well this is more or a mixing it up thing. You work it out.
8. Same IP blocks. If you always put your sites on the same IP start looking at new Cat C’s (or whatever it is called these days). You have seen the IP command in MSN I presume. That’s what we have access to. To be uber safe maybe try and hunt down different Cat B’s.
9. Stupid linking. Linking back to one source or even worse heavy cross linking could get you dinked. You need to ringfence those suites of sites, so if you do get a direct hit, you only lose a %, not the whole lot.
10. CSS naming conventions. Always using the same naming id’s and class’s could get you a good shoeing. The argument that lots of people may use them and they can’t kill everyone is not a good one. Why? Cos they don’t just nuke everything, they could build up a pattern, a number of black marks, to piece together the whole story, then you get a kicking.
11. Same onsite monetization. Yes sites need to make money, but making $2 a week for some cack feeder site and losing a whole network is not worth it. If you are daft enough to put Adsense on all the sites, at least make sure you have several accounts. Which is not that easy and requires serious discipline not to let them know.
12. Crazy surfing patterns. What do you check out on a daily/weekly basis, do you do nutty things like SERPS checks using WebPosition Gold? Or maybe other things like flick home to Google after looking at your sites with the Toolbar installed or being logged in. Whilst they say they may never use that data, it was a long time until MediaBot was acknowledged as doing some indexing. To me that always seemed obvious, but what do I know :-).
13. Avoiding others peoples footprints. Keep this one short, if you buy links, how good are they. Are there lots of other links going from the pages to 100% different things? Best maybe not take those links, you could get entwined in a web of shit.
Ok, well that’s it for now, there are lots more things to look out for, but not sure if I want to out them, as I would like to keep something back. You could always offer me money to tell you.
Cheers for now
