Originally Posted by stedalus

Velox makes rubber handlebar plugs with screw expanders. LBS should have them or be able to order them.

But, usually it's just a sign of someone doing a bad job on the wrapping. There should have been about half an inch sticking out (or more if it's really thin) to grab on to the plug. You could try electrical tape as suggested above, but I think a short piece of cork tape would work better.