One of my lenses I always carry (Olympus Pen lens, I fit it on a Sony NEX), has a focus ring that is becoming a bit too loose. Probably due to carrying it in a bag very often and taking a lot of pictures.
What can be done to fix this easily / cheaply?
Personally, I'd at least ask for a free quote from Olympus, especially if this is a lens you use frequently. It may be much cheaper than you anticipated and would be a more permanent solution than trying to fix it yourself with glue and such. Here's the website repair page.
I shoot Canon and was hesitant to send in a lens for repair because of the cost, but it made a big difference in the end.
Maybe you could get a wide, heavy duty rubber band, and put it over the join on the lens barrel. If it's the right size, then it should provide enough friction to stop it from turning. You could even apply a bit of glue to one half to keep it in place.
Something like this.
