Ditto to capnjohn - be sure to track your maintenance costs including the labour for various tasks so you know whether your $0.30 light bulbs are costing you $100/yr in extra labour costs.
When we began our PM program from scratch, we formed a team of engineers, techs, and ops people to sit down and brainstorm everything on the property that could conceivable require some sort of maintenance.
We then assigned scores for ramifications of failure (production criticality, the number of dependent systems, safety, environmental), a subjective assessment of the likelihood of failure (how robust the system was), a subjective assessment of the likely downtime (did we have spares, good documentation, drawings, etc).
After that, we came up with an aggregate ranking scheme that gave each system an overall priority score so that we knew where to start. Otherwise, you don't know whether you're investing your labour in the right places in the early parts of your program.
Document your decision matrix so that everybody can understand it and then you can modify it as needed.
Good communications and planning is critical to a successful maintenance program