In Theory:

William Zartman who coined the term "ripeness," describes what he means.
William Zartman gives examples of situations that were "ripe" for intervention.
William Steubner suggests that even after has a conflict has become ripe for negotations, it may be difficult to "sell" to conflict profiteers.
Roy Lewicki considers whether facilitators should pursue resolution when the situation is not "ripe."
Richard Rubenstein talks about the role of external actors in helping parties to get out of a hurting stalemate.
Jannie Botes explains that journalists frequently escalate conflicts. This can be positive or negative, depending on the situation.

In Practice:

Mediator Frank Blechman often gets called in when conflicts are "grid-locked."
Carolyn Stephenson explains how the "carrot" of EU membership helped push Greeks and Cypriots into negotiations on Cyprus.
Andrea Bartoli talks about "ripeness" in relationship to the resolution of the civil war in Mozambique.
Elise Boulding on getting in the disputants' frame of mind.
Leo Cardenas explains how strong emotions are important to get people interested in working on a problem.
Nancy Ferrell explains how empathy is necessary for successful relationships and successful mediation.
Nancy Ferrell explain why it is so important that each side sees the legitimacy, at least, of the others' interests.