Get some of those springy things or rubber ball that people squeeze on to strengthen their grip. I recently started getting a little bit of numbness near the end of a two hour ride and nothing had changed. I got to thinking that maybe I was just loosing muscle in my hands since I'm semi-retired and not doing as much with my hands anymore.
Actually seems to have helped. But it took about three weeks of daily use before I started to notice anything. And a recent 3.5 hour ride was no numbness.
Any of the other things mentioned are good things to try too.