The Fasstco bar-end inserts do a pretty good job of damping vibration. Also, I've heard about guys filling the gusset area behind the steering stem with foam, because that area tends to transmit vibration to the steering head, then to the triple clamps, then to the bars, and to your hands. Anywhere you can damp the vibration along the way, you're going to make an improvement.
Although I have not tried it myself yet some of the other blokes with KX 500s swear by filling the bars with silicone sealant (at the moment the end plugs on my barkbusters are being awkward and I don't have time to bother with it)