My A/C vents kept closing on their own when the A/C was running. It was very frustrating as every few hours I'd have to go around and open them again. I was worried about them closing and freezing up the compressor. I had it to the dealer and they "adjusted" them (not sure what adjustment there is!) but they still closed. Below is what I did to fix them.
First, I removed the vent by removing some of the louvers and then pulling on the edge. I then put a dab of caulk on the holes that hold the smallest louver. This creates just enough friction that the louvers still open and close, but only when you push on them. To finish things up, I sprayed some lithium grease on a paper towel and rubbed it in the opening so the whole vent would turn to direct the airflow. Some of them had started getting very difficult to turn.
I can now sleep all night, and in the morning the air is still blowing on my face!