1. Regardless of what management wants (or you think they want), put exactly what you want to do in your start of year planning. Make your manager change it. Note in the document that it was changed and whether you agree with the change or not. (If you don't get to put in the document what you want, why are you filling it out at all?)
2. If, in the next year, you are asked to work on something not in the document, treat it like a change of scope/requirements. Make sure the document is updated and the change is recorded.
3. At year end, know why you've successfully completed or failed to complete your plan. Save emails.
4. Basically, treat your job like a development project.

If nothing else, you get to make statements like, "By adding this new responsibility to my performance plan, I would have 11 objectives," (there are always 10 blanks for objectives aren't there?), "In order to fairly assess my performance alongside my coworkers, it seems we would either need to remove one of my objectives, or give everyone else another."