I would say the protocol is exactly the same as asking for any location: Know exactly how many days and for how many hours per day you will need the location. Know exactly how many people will be there and exactly where they will be parking. Know about your power consumption and where you're getting power. Know exactly how you will be protecting the location. Know exactly what support vehicles and equipment will be on the location.
Since you are asking for a location for free, remember that someone will need to be the representative (security guard, manager, owner) of the location and that person should be compensated some way. Often if you offer to pay for that persons time - at their usually hourly rate - a location owner is more likely to donate the use of the space. If you need to push your shoot for a month or so while you earn that money, you should.