Where is the best place to park a folding bike?
if you can: carry it with you. When I was riding my brompton to school, I'd just fold it and take it into buildings and classrooms with me. There were some building guards that didn't like bicycles, so a small fabric bag/cover was used to disguise it.  The brompton is unique in being small enough to tuck in just about anywhere, even under a conference table. Since it was by my knees, I didn't bother locking it up. 

I have a Dahon at my parents' house which is bigger, heavier, and not easily carry-able. I locked that bike up at bike racks with a solid lock. It's hard because many folding bikes lack good places to lock because they aren't  double diamond design.