For centuries, my 280Z has been plagued with creaking plastic interior panels.  I attempted the foam strip trick, the silicone trick, I tried dabs of grease, and I tried armor all and WD40.  Some work, some don't, non of them worked very well for very long.  I took off the panels and went for a quick test drive and what a difference.  The car sounded solid with almost no interior noises.  With the plastic panels laying in the yard, I got an idea.  Rubberized undercoating.  I have a can of Car-Quest brand which goes on very thin and dry.  I just coated the backs of all the panels with a light coat.  I will keep you guys in the loop.  Maybe we will FINALLY have a cheap and easy answer here. Â
EDIT: Oh yeah I discovered that you need to grease the plastic retainers a little bit too. Â They tend to creak also. Â