Here's the rules that I use, the system is that you have to match at least one rule from both sections A & B.
Section A (basic requirement)
I am interested in you.
Section B (additional qualification)
I have your number in my phone.
We've lived together.
We've been educated together.
We've been on holiday together.
We're colleagues, but I see you regularly out of work.
I want to stalk you.
At some point I'll try and get round to brutally enforcing these rules on my current friends list and post the difference.