I had this same problem for a while when I first got my new bike. Two things I did seemed to fix it, although I am not sure which was a bigger factor. I used "bar phat" gel pads when I retaped my handlebars, and I tilted my seat up to a more level position. I looked and realized that my seat was tilted slightly forward. I think levelling it probably made the biggest difference by not placing so much of my weight on my hands, but the bar phat definately dampens vibrations through my hands. I also rotated my handlebars up slightly so that the hoods were higher on the curve, thus allowing me to ride the hoods without digging the upper curve of the bar into the side of my palm. Doing those three things stopped almost all of the hand numbness I was getting. I was like you in that it didn't go away when I got off the bike. Even a 20 mile ride over moderately rough pavement could cause numbness that lasted a day or more. Now, just 4 weeks after the modifications described above, I was able to ride 50 miles on various pavement types in a day without any numbness at all. Good luck and hope that helps.
