Something that has worked well for me in the past:
I am taking it that if your project manager wants to sneak in features they do not have an accurate project plan. Keep your own task list with an estimate of how long each task will take, ordered by priority - it doesn't need to be elaborate just a text document or spreadsheet. If your project manager wants you to add a new feature, send a copy of your list and ask where you should insert it in the priority order.
If the project manager tries to negotiate down your time estimates then just say "I will do my best, but I can't guarantee anything."
