My beloved 24-105 f/4.0L is the lens most showing it's age right now. I have to keep a constant grip on the zoom ring. If I have it pointed upward, it "falls" to 24mm, if it's pointed down, it "self-zooms" to 105mm. However, image quality is still great, and it gets used pretty much daily.Take a rubber band from a head of broccoli, place it on the lens at the joint of the zoom ring and the lens body..... it gives extra friction and that stops the lens creep.
Hmmm. . .
I'm a dedicated improviser, but that's one I didn't think of. Props also to Brad-man for the silicone band idea. My wife had one of those laying around from a fund raiser at her work some months back. I worked it into place on the lens and no more "auto-zoom." Makes it a little stiff to turn by hand, but it beats the alternative.
All this reminds me of the vacuum cleaner drive belt some of us used to augment the skinny, undernourished focus ring on the original Nikkor 180 2.8 AF lens back in the late 80's. (Yes, at one time I had an inappropriate relationship with a pair of FM2's. I was young and I needed the money. . .)