Focus on IP address design and management. Start the IPv6 prefix assignment application process now. Stop worrying about conserving addresses and start thinking about adding meaning to individual hex digits. And ditch the spreadsheet to track all this.
Update network support systems Do you have an internal DNS infrastructure? Can nameservers support both IPv4 A and IPv6 AAAA records? If they're dual stacked, how do they respond to a name query when there are both IPv4 and IPv6 addresses assigned?
Budget for security updates and expertise End-to-end IPsec notwithstanding, security systems tend to be the problem children in IPv6 deployments. Not everything will survive the transition, so allocate some funds here.
Understand the lingo Tools for monitoring, logging, alarms, configuration management, and change management have to understand IPv6, not speak it.
Have end-to-end training Don't limit IPv6 education to IT. Going all-IPv6 positions your company as a technology leader. Make sure customer-facing personnel can tell the story.