It shouldn't move much, right? If so, maybe using a commerical grade receptacle instead of the cheap 99 cent receptacles will give it enough resistance such that it won't fall out.

I find that the commercial grade receptacles 'hold' the plug much tighter.

greg's solution is what I would try first, but you could also supplement it with a small nylon cable clamp for the cord that you would tightly screw to the ceiling.

Nylon cable clamp

Use a clamp whose closed diameter is slightly less than the cord's, and mount the screw in an anchor if you can only attach the clamp to sheetrock. Done right, the clamp will take the strain and the plug will remain in place because it's not being jostled

Greg, Good suggestion, and I may yet need to change the duplex outlet. The licensed electrician was supposed to install commercial grade outlets but it's possible that he didn't.

Kudzu9, Your suggestion was easy for me to try since I had a number of wire ties and some tie hooks. That's what I did. If it doesn't work, then I'll check with the electrician and replace the outlet. It is tied into the two fluorescent fixtures, so I would prefer not to unless necessary.

I took some photos, but the photo tab below this reply doesn't seem to upload them.

Thanks to both of you for helping me with this dilemma.

Use a twist lock plug and receptacle. Or a strain relief grip like these on the wire.

Twist lock is an option, but I always hate to cut off a molded, original plug unless I have no other choice.

Elitwo and Rwiegand. What excellent recommendations, and I had no idea that either of these options existed! Since I've already tried Kudzu9's suggestion, I'll see how it works. If it doesn't do the trick, then the twist lock plug and receptacle, the strain relief grip, and the commercial outlet options will be the next ones I try. Thanks very much!