(I advise you not to read much beyond the first paragraph of the Wikipedia articles if you intend to read the Dune series. I did, only having read Dune and I regret it.)
Mentats are humans with supreme calculating and analytical powers. They were bred to replace the AIs that were overthrown and stamped out in a furious galatic jihad 40,000 years into the future, 10,000 years before Dune. But that hardly makes them merely human computers. They are better than that. They can literally take in all the information given to and observed by them, sift through those large volumes and grip the various logical paths and choose the one best suited for the current situation. They are the ultimate counsellors.
At this neat little page is a full and detailed list of the qualities of a Mentat. I can easily predict that us, as students, would see even one of these qualities as a godsend unto us, lightening our workload by many times. I aim to get all. Not now, certainly, but one day I hope I can have them. Here is another page with many more details about the techniques involved, but I've just found it today myself and have yet to read it, let alone test it.
Mentat Techniques (taken directly from linked article)
- Extraordinary capacity for conscious information acquisition
- Rapid computation skills
- Near total recall
- Unshakeable concentration
- Calculate human or social costs without emotion
- Wide-ranged analytical talents
In the Dune series they are feared as witches whose only goal is to control the human race. This is what they do, but their true goal is to manipulate and shape the interstellar politics of the Great Houses of the human Imperium. This is done such that after centuries of marriage and conjoining of bloodlines a male BG, the Kwisatz Haderach will be produced and unlock the ancestral male memory, marking the maturity of the human race. He is produced, but turns out to be both far more and less than what was expected of him, most of which in that he was a generation early
Aside from power manipulation, the BG also specialise in unarmed combat, espionage and intelligence work. I will not personally post much more about them, since information on them is even more than that of Mentats (and I can't understand much of what they do >.<), but here are two links at Dunepedia and KnowledgeRush.
As a last entry here is the litany Bene Gesserit sisters recite to themselves in times of stress to calm themselves. Myself, I find it is not only the words that work, but the rhythm of the phrases.
Fear is the mindkiller. Fear is the little death that brings obliteration. I will face my fear. I will permit it to pass over me and through me. And when it has gone past me I will turn to see fear's path. Where fear has gone there will be nothing. Only I will remain.