Strut cover

I can't figure how to get this thing to stop moving around. The rear ds strut's cover(the little plastic acordian looking thing that goes around the bar that comes out of the strut), keeps popping out and then the strut will start to "squeek." If I spray some grease on it and pop the cover back down it will be good for about a day and then proceed to pop back off and start squeeking again. I'm assuming the squeek is because it's drying out or getting debris in it. Any way to keep this thing in place?