I have had excellent success injecting Botox to alleviate several different types of nerve pain including nerve pain from shingles ("post-herpetic neuralgia").
A study in the December 2010 issue of Pain Medicine showed that the majority of patients receiving this treatment experienced a decrease in overall pain scores, improvement in sleeping, and a decreased need for pain medication. The effects lasted for at least 3 months.
If you chose to pursue this treatment, be sure to find a physician with lots of experience. As I am aware that post herpetic neuralgia is a significantly debilitating problem, I wish you the best in finding a solution to eliminating your pain.