As mentioned above, I would look in South End, or along Morehead and Graham St. You can drive up, or book in advance online. Avoid garages if you want to set up a tent or grill (even on top).
You can buy passes in advance online through preferred parking, parking panda, etc. I've used 3 or 4 different ones, and never had an issue. I usually get out there before the parking attendants even show up (they usually show up around 9-10). I'm usually set up by this time, and they haven't given me any trouble when I show them my pass.
Most of the lots say they open 4.5 hours before gametime, but this is not enforced, and most are open by at least 10am.