Brownfields/Voluntary Cleanup Program

Curious if anyone has any experience/lessons learned about a heavily contaminated site. Obviously the best strategy would to be avoid it unless you have some experience with remediation/abatement and mitigate all future liability to the extent possible. But apart from that, any suggestions about how to manage environmental consultants/good questions to ask them or local authorities? It seems like even if you complete a Phase II and have a sense of what you're getting into, there are still lots of unknowns.