Thanks to Herb for listing his in another thread and getting me thinking about it.
Since, in the ten years of doing this, I've never had a previous SB return to the fold, mine are a bit different:

 
1. Never pay in advance (the corollary, never loan money or buy autos, apartment, etc).
2. Never overlook (dismiss) a major personality/behavioral issue in the hopes it will go away
3. Never seriously argue with a SB, you will undoubtedly  win the argument but lose out on the pussy

 
Interesting to hear what others think.