I bought a camera with an old zoom lens where the zoom ring would move under gravity. I could have tried to disassemble the lens, but there was a good chance I would have destroyed it, so I opted for a simple external procedure combining two different ways of placing cardstock/tape shims to add friction to the zoom ring.

I do not recommend doing this with an expensive lens. If your shims come off and get stuck, the lens could be destroyed. I did this on a lens that was included with a $20 ebay camera.

Supplies

card stock
sticky tape

Step 1: Folded Cardstock

The first method is this. Cut a small rectangular piece of cardstock (e.g., from a cereal box, etc.), maybe 8mm wide. Fold it in half so it's closer to square. Slide it so one folded side goes under the zoom ring and the other on top of it. Holding it in place, move the zoom ring through all of its range of motion to make sure that the cardstock doesn't get caught on anything. If it does get caught on anything, try a location at a different spot on the zoom ring.

Once you've found a safe location, tape the outside portion to the zoom ring.

Step 2: Unfolded Cardstock

Cut a rectangle from cardstock, not too large (e.g., 8mm wide). Pull the zoom ring out to expose as much of the lens housing under the zoom ring. Slide one end of the rectangle under the edge of the zoom ring, and then thoroughly tape the exposed part of the rectangle to the exposed part of the lens housing, so that if you retract the zoom, the rectangle will be under it. Now move the zoom ring through its entire range of motion to make sure that the shim doesn't get caught on anything. If it does, try a different location.

You can adjust friction by adding more layers of tape.