Rule 1: The object of the exercise is to negotiate. This does not mean trying to low-ball and not moving for the sake of trying to strike a hard bargain. In truth, the other side would just walk away. So, negotiate!
Rule 2: Compromise - Negotiations aim to get a result that both sides are happy with. Be prepared to compromise. Problem solve, find creative solutions (this is what the exercise is testing). As I mentioned above, it is not an exercise of getting the best deal, it's about showing how you personally come to a deal.
Rule 3: Despite the above, don't just capitulate. You still want to try and get a good deal for your client. But, there are ways to go about doing this.
Rule 4: There is a good way to work out how to "pitch" your opening offer: Find your basic realisable alternative (aka, if you have another offer, that is it) "POINT A". Then, work out what you NEED to achieve "POINT B". Guess what you think the other side CAN offer "POINT C", then pitch slightly above "POINT D". Your aim is to reach an agreement between POINT C and POINT B. If you can magically get POINT D, great, but that should technically be impossible as the other side will have been expressly told to not exceed POINT C.
Rule 5: Be friendly! Smile, be open, be willing to negotiate. Remember, this is a point-scoring exercise, not an exercise in playing tough