I compiled a list of tips that worked for growing out my horse's tail here: http://paperchasesandpetticoats.com/health/9-tail-care-tips. This is one case where less can be more: the less you handle the hair, the more precious strands you’ll preserve.
Products that are ingestable like SmartPak products are effective. Additionally, products like MTG if applied daily for 3 weeks, may start to grow out a tail or any patch of skin on a horse! Hope that helps!