For controversial or difficult projects, turn potential enemies into allies by including them in the project team.
They may come up with better ideas to get the project done, and they will then take ownership and work to convince other enemies of the project that it can work.
Environmental concerns are a big one here. If you can include the client or other stakeholders in the process (not just the contract negotiation project review), you will be more likely to get acceptance.
Don’t exclude input from people who have a say in the approval of the project.