I have a pair of entirely plastic framed glasses. When looking down at things (especially over the side of a pitching boat, facing the sea), these glasses would start to slide off my face. They fell off a couple of times, so I put a block of Sugru on the inside of each arm to make the arms grip my head better.
This worked very well, for a time, but the plastic arms were so shiny that the Sugru eventually just peeled off.
I decided to have another proper go, but I'd run out of Sugru in the interim, so I just used hot glue. This instructable is pretty simple, but I guess it might help someone, and I couldn't find another example of it.
Step 1: Remove the Arms
Step 2: Cut and File the Arms
I used a round rat-tail file, and a hacksaw. You could use anything that will scratch or cut the plastic, like sandpaper.
Step 3: Apply Hot Glue
Tip: Start with a thin "blob" of glue -- you can always add more later. I tried blobs around 2mm thick first, before bulking them out to 4mm.
Step 4: Reassemble
Then, your glasses are ready!