2) Determine what the other side's must-have items are as well. Do this by asking them what are the most important things to them. Win-win scenarios happen when their must haves are something you can give up while your must haves are something they can give up. The result is that both of you are better off.
3) Invoke previous negotiations when the other side had agreed to similar proposals. Most people want to be fair and consistent.
4) Tell the truth. This isn't only about having integrity but about pragmatism. Being honest is the only way the other side could know what your must-have items are, and it would encourage the other side to do the same. If you cannot divulge certain information, then simply say you don't want to go into details. Furthermore, if the other side sees and confirms your honesty and integrity, the lowered risk for them allows them to more likely and quickly accept your proposals.
5) If the suggestions above still don't work, then go ahead and concede defeat to your two-year-old. But for everyone else, keep the relationship positive and come back again later when factors and circumstances inevitably change. Timing, as Sun Tzu can attest, is everything.