My stock bar ends suck. They're tiny, and aluminum. To help damp vibration, here's what I did:
I bought some 1/2" rubber tubing, some silicon caulk, and some BBs (lead shot works too).
I measured how much tubing would fit in my bars (about 8 1/2") and cut the tubing to fit.
I plugged one end of each tube with caulk and let it dry.
I filled the tubes with BBs and plugged the other end with more caulk.
Once it's all dry, I slipped the tubes into my bars and put the stock bar ends back on the bike.
The whole thing costs about $10 to make youself, and it helps quite a bit!
Eventually, I'll get some big & heavy bar ends to add to the mix, to dampen the vibration even more.