Negotiation is a good way of settling issues before they go out of hand. If you see that you are involved in an erosive war, try to negotiate for some truce or make alliance with someone else to get rid of the enemy. Do this soon, or nobody will bother to get allied with you and they just wait to see when you two become weak enough that they can take you. Bottom line: erosive war is bad and should be avoided at all costs.
Secret negotiation is probably the best type of diplomacy in the game, but it is almost never possible (unless you play through a computer). If you are playing Risk with your friends sitting around a table, then if you want to have a secret negotiation or alliance with somebody else, whether it succeeds or not, other players will assume that you two have some sort of an agreement. They might all go against you just because you talked in secret and probably behind them. This makes the rest of the world your enemy. This is bad news!
Sometimes you can initiate negotiations before even the attacking stage of the game starts. In the beginning of the game, you may still be wondering which continents to choose, and another player might also be wondering the same. Start a passive negotiation by telling him how nice it would be if you had a deal and so you did not have to worry about each other. Right from there you have an advantage over others because you can focus on them and increase your number of armies in their borders. This way no armies would be misplaced or wasted for some useless attack.
Deception and negotiation are thoroughly discussed in the book.