A collection of maxims relating to privacy might be interesting as we seem to have some common principles that fit well to this developing. Anyway here are some that come to mind from my work:
Don't collect what you don't use
If you have no use for the information you're collecting from the user, or can attribute no value to it in terms of the product being presented, then don't collect it at all.
If it looks like PII, it probably is PII, so treat it as PII
PII doesn't just mean identifiers but can extend to other kinds of data, eg: device identifiers, session identifiers, tracking etc...if you can map a set of data to a single unique person, or even a small, significantly small group of persons then it probably is PII.
Don't shock the user
You collected what??!?!! You shared what without my permission?!?! How did you learn that about me??!?! I didn't agree to that??!?!? T&C's...I didn't understand them, let alone read them...and you still did 'x' !! Enough said...
Location data isn't just GPS co-ordinates
Maybe this could be rewritten as "Never underestimate the power of geolocation functions"....it however does come as a surprise that many types of data can be transformed into locations, even if very granular...who needs a GPS receiver anyway?! Pictures often contain vast amount of EXIF data including locations, mobile phone cell IDs can be mapped to precise location and triangulated for example...
Good security does not mean good privacy, but good privacy doesn't come without good security
If your system isn't protecting the communications and data then it doesn't matter what you do to obfuscate or anonymise the data. Actually getting the balance between these sorted requires some serious engineering competence - security and privacy engineers are few and far between.
All information can be transformed and cross-referenced into whatever you need.
Need to think about how that one is written but whatever data you have can be transformed (eg: extracting usernames and other parameter data from URLs, meta-data extractions etc) and cross-referenced...
This one is more of an apophthegm, and a reminder that we need to serve the users not ourselves:
Security through Obscurity, Privacy through PowerPoint and Policies...
See Jim Adler's talk at #pii2012 on the Emergent Privacy-Industrial Complex - a term coined by Jeff Jarvis in an article called Privacy, Inc: Scare and Sell. But seriously we forget that ultimately privacy gets implemented through good, sound software engineering in C, Java, Python etc. Getting privacy as an inherent part of system engineering practice (not just processes) is probably the most critical thing to get all those policies, laws and good intentions implemented. As Schneier likes to use the term "security theater", I think we can often use "privacy theater" to reflect many of the things we are doing, see: Ed Felten's posting on this on the Freedom to Tinker site.