I think a birth plan is useful so that you can think through the pros and cons of various options, and absolute no-nos so that you don't have to take in lots of new information and make a decision in a crisis. And while you can't have control of how your birth goes, you can control the environment to give your body the best chance of getting on with labour.
So my birth plan (for a home birth) was roughly:
No routine internal examinations.
If there are any internal examinations, don't tell me how dilated I am.
Don't offer pain relief, but have gas and air available if I ask for it.
If you think I am likely to need to transfer to hospital, let me know asap, as I would prefer to transfer at as early a stage as possible.
Keep cord attached until after placenta has been delivered.
Placenta to be delivered naturally.
Skin to skin with me immediately after birth, and baby to be allowed to crawl to the breast.
If skin to skin with me is not possible, skin to skin with the baby's father.
Do not give the baby formula milk. If supplementary feeding is required, please use expressed colostrum/donor milk from x, to be given by syringe/cup/tube as appropriate.