I agree with this post almost entirely, especially the section on sob stories. You want to convey professionalism and knowledge of the discipline. I would also add that sociology SOPs are different from political science or other disciplines. You really need to get a sense for what is typical/what the admissions committees want (for example in history they want a very specific research agenda - almost like a research proposal - whereas I recieved feedback that my statement was too specific in earlier drafts for sociology). You don't need to be married to this proposal - the admissions committees know that you research agenda may (and probably will) change over the course of your PhD - you are signaling with this your ability to think like a sociologist and knowledge of the discipline. I think a good general format for these things are introduction (why sociology - which will probably include some personal stuff), your background, research agenda, why you want to go to that specific school.
The one point I disagree with is that you should not edit your writing samples. This is not the advice i got - you want to put your best foot forward. If you were worried about submitting something that was far afield from what you originally submitted for a class, you could always explain on the first page header that this is a paper based off of a paper you wrote for xyz class. I agree that it's really important to be honest with the applications (as in anything in life!), but it's also important not to submit something riddled with spelling errors.
One thing I've realized through this process is that (among many other things) access to the information required to make a good application is very limited and unfairly distributed. I went to an elite private unviersity and got all the advice that Alasdair so kindly put together and published well before my application. I guess what's great about this site is that we have an opportunity to spread more of this information, although it's always so tricking putting things online. Good luck everyone.