Having been in the same situation (of giving negotiation advice) many times, I tried to step back and summarize what I think I know, especially in light of how my advice has been received and when, in retrospect, it has proven useful. First, negotiation advice needs to be short and sweet. (Maybe ghe same thing is true with regard to advice on any subject.) When the presentation gets too complicated, the whole message just washes over the recipient. Anything more than a handful of power point slides (if you use them at all!), is too many. I would suggest starting with a very short statement of the negotiator's problem as you understand it and the gist of your advice in just a few sentences. I would look right in the recipients eye so they are convinced that (1) you understand the problem and the pressures they are facing; (2) you empathize, and you've thought long and hard about what you are going to say; and (3) you have confidence in what you are suggesting. The student teams I watched, divided up their presentations so that each member of the team had some air time. Don't do that. It is hard enough establishing personal contact with the person you are advising.
Second, don't try to explain the theory behind your prescriptive advice. Again, this is probably true of any kind of advice, not just negotiation advice. How I reached my conclusions is my business. If they are asking me for my advice, they don't need to hear everything I've thought about and everything I know, just my conclusions. Negotiation advisors who are not confident about the advice they are giving are likely to share everything they know with the people they are advising. In my experience, that just makes recipients uneasy.
Third, stop along the way and make sure that the person you are advising understands each of your key points. When a recipient get's stuck on something I've said -- if they don't understand it, or it doesn't sound right to them -- they stop listening. So, it's better if I break what I'm going to say into a few short pieces, and then check to be certain they've understood each point before I go on.
Fourth, emphasize the possibility that additional value can almost always be created, and suggest ways of reframing a negotiation in terms of all-gain solution. Most people entering a negotiation are likely to be thinking in zero-sum terms (i.e. whatever my negotiation counterpart gets, I lose; and, vice versa). A skilled negotiation advisor, however, is ready to point out ways that more value can be created. This can take a lot of pointed questions and several rehearsals. Nervous negotiators are often unable to hear the kinds of questions that lead to "all gain" solutions. For example, I often press people seeking negotiation advice to put themselves in the shoes of the person with whom they are about to negotiate: "What do you think the key interests are on the other side? What's most important to them? What can you offer them (at low cost to you), in exchange for things you want from them?" In other words, I try to frame my advice in terms of possible trades or packages that will be mutually advantageous. And, I ask the person I'm advising to play the role of their negotiation counterpart as we search for value creating possibilities.
Fifth, it's OK to offer contingent advice if there are major uncertainties or assumptions which, if handled differently, would lead to different suggestions. I think this is especially relevant to negotiation advice. While I'm trying to keep my suggestions as compact and simple as possible, it is, in my experience, appropriate to say: "If X turns out to be true, as I think it will, then my advice to you is this. However, if Y happens instead, then my advice would change in the following way." I wouldn't do too much of this, but one or two contingencies will add to, rather than subtract from, your credibility as a negotiation advisor.
Next, a diagram can be useful. But elaborate power point presentations or complex conceptual schemas are a distraction. Diagrams summarizing prescriptive advice need to be anchored to real events, dates or steps that make it clear the order in which things need to be done. Elaborate conceptual diagrams, spelling out theoretical ideas or summarizing various schools of thought, should be avoided. Someone seeking negotiation advice wants to know the most important things they need to do. That's it. Negotiation advisor need to be ready with short explanations of "why" if they are asked to justify a particular move; but, these should be held in reserve, and offered only when requested. And, "why" answers should always explain the dynamics involved, not the inspiration for what the advisor is suggesting. Why answers are not like footnotes! They should expose an additional layer of understanding, not cite a source. So, for example, a suggestion like, "Don't share all the information you have on that subject until you know whether you can trust the other side," should be followed with, if asked why, "Because they could use that information to "anchor" in the Zone of Possible Agreement at a point that is best for them and worst for you."
I worry about the increasing use of power point presentations in advice-giving situations. I think it is a mistake. It puts a barrier between the advice-giver and the recipient. Rapport is everything, and power point presentations, especially those with glitzy special effects, get in the way of personal connections. I don't mind a one page handout (that the recipient can annotate as they listen), but it should not be filled with dense text. Just a few bullet points will do.
Let's turn the tables for a minute. If you are the one seeking negotiation advice, how ought you to frame your request for assistance? I might ask: "I'm heading into a salary negotiation with a prospective employer, and its making me nervous. I don't want to lose the job, but I also want get a fair salary. What's your advice?" In this situation, I don't want general negotiation advice. I want specific suggestions tailored to my salary negotiation. I want my negotiation advisor to acknowledge my priorities and my state of mind: I don't want a prospective employer to give someone else the job. But, I also don't want to settle for less money than I deserve. A skilled negotiation advisor knows how to ask for advice, not just give it.
The things unique to giving or getting negotiation advice (as opposed to any other kind of advice) are: (1) general rules, and the theory behind them, are a lot less helpful than suggestions tailored to the specifics of the situation; (2) sensitivity to the way an advice recipient is feeling may be as important as any substantive insight an advisor can offer; and (3) it is important to emphasize the possibility that additional value can be created and almost every negotiation can be reframed in non-zero sum terms.
How negotiation advice is offered is as important as the substance of the advice. If negotiation advice is offered in the wrong way, it is not going to be very valuable.