The most convenient tailgating location is in the Alamodome parking lots. But like mentioned above, the issue is getting parking passes if you're not a season ticket holder.
Some of the private lots around the Alamodome allow tailgating. I don't have a lot of information on those areas. They're owned by private individuals and they make their own rules.
If you're able to get a hold of an Alamodome parking pass here are a few things go know:
A parking pass is only good for ONE parking spot. It's open parking.
If you're going to have a grill, make sure you have a fire extinguisher.
The Alamodome parking lots have port-a-potties and a police presence.
There are three Alamodome parking lots, A, B and C. Lots B & C are the major tailgate areas.
More detailed info can be found at the bottom of this webpage: http://goutsa.com/news/2011/12/5/205342663.aspx