Don't forget about bike fit...
Clearly getting good shorts and a good fitting saddle (the Selle Italia Prolink works for me as I have wider "sit bones"), but don't forget about the other measurements on your bike, like reach and drop. Chances are you may have a little too much of either or both and are leaning forward too much on your bike.
I used to have issues with my Trek OCLV as it was just a little small for me. I had a fitting done and found out I needed a bigger bike but a little less reach than I was running. After the fit I also found out I had my saddle tilt just a little off. Once adjusted, I rarely have any numbness other than on really long rides. A HUGE difference.
I suggest you check out a LBS that does bike fitting (like the Serotta fit program) and let them help you. It will cost you $90-100, but well worth it in the long run.