I used to have numb hands.
It started out that it would happen only every so often, but then as I increased my training time and distance it would happen every time I went out.
I sent out a facebook discussion on the matter and received so many comments and suggestions and I tried many of them and finally resolved the problem.
I biked 60 mile in October for the Viva Bike Vegas without my hands going numb at all.
Thanks to everyone who helped me with this.
Here are some of the things that I tried.
I bought new gel padded bike gloves, mine were wearing thin.
I changed the angle of Ergon hand grips, flattening them out to hopefully relieve any nerve pressure. I also raised the height of my handlebars back up as I had previously lowered them completely. I made a conscious effort to hold lightly on the handlebars instead of gripping them tightly.I made a very conscious effort to put my weight into my legs instead of leaning or resting down on the handlebars while riding or coasting, one of the exercises that was suggested to me that i practiced regularly was to only ride with one or two fingers touching the handlebars therefor forcing my legs to bare my weight and my fingers were for balance only. Another trick that was suggested was to do quick switches. Only kick 3 times on each side before switching legs. All of these suggestions were great and helped to an extent,
but it wasn't until I got a new short stem which brought my handlebars so much closer to my body that my numb hands went away entirely.
If you suffer you may want to look into a new stem.