Devise a plan, preferably with multiple goals. Choose not to use the internal system. Make others play by your rules. The advantage is that they will not know what your rules are and you have the advantage of changing them at any time, giving you the flexibility you need. Hold others accountable on your terms.
Surprise is an effective technique.
Timing is everything. When going into a meeting, go in well-prepared and with well thought out strategies. Don't be pressured into making any decisions you're unsure about or uncomfortable with. If you feel a meeting will be or may become adversarial, bring someone as a support person who will also take notes or inform all parties that you will be tape recording the meeting. Excuse yourself politely and leave if things become too heated.