Identify technical experts who are credible to all concerned parties. They provide the objective technical information. Early in the process, give this information to decision makers. This allows everyone a chance to review the information and ask questions. When the time comes, the decision will have been made after careful consideration, and not on the spur of the moment.
Communication is one of the keys to success.
In local governments, the staff who implement policies are frequently consulted by decision-makers. The staff is also a good source of advice for citizens. They can point out major obstacles or opponents. Overcoming the obstacles may mean doing research, answering questions, or meeting with people. But, once this is done there is a much greater chance of receiving approval.