I did quite a bit of tanking in WoW, along with a fair bit of healing. One of the most important personal rules I followed hasn't been mentioned yet. If the tank consistently gets to 100% health during a fight, the healer is probably over-healing. Keeping the tank just below 100% is optimal. Over-healing increases hate towards the healer, making the tank's job more difficult. There will be times when an over-heal becomes necessary, but try to avoid it when possible.