Short Answer: I put a loose "candy cane" spiral of electrical tape on my bars, under the grips.
A few weeks ago, I got tired of my ESI grips slipping on my bars and went looking for a solution.
This post said that putting stickers under the grips fixed the problem. Intriguing. But I like things tidy, so I went with a spiral of electrical tape instead. It's only been a few rides, but so far that's done the trick. Zero slippage.
Once my ESI grips wear out, I plan to switch to RedMonkey Kärv(xt) grips. I should get zero slippage, a comfortable grip, and RedMonkey supports local racing. Nice!