Here's an excerpt from my notes from a talk by Khenpo Karthar Rinpoche that I thought was worth posting. He was teaching on Lamp of Mahamudra. A book has been promised, but not yet published, so until then my notes all I've got.
Primarily what is the cause of all the mistakes of meditation is that we have not purified our defilements and gathered the accumulations. So doing these will purify all of our mistakes. To get to the point of recognizing mahamudra you have to meet with all the conditions that bring it about. It is like growing a crop. To practice mahamudra, all the other conditions have to be present. The great Drukpa master I mentioned yesterday was named Dogchup. He made his students do ngondro sixteen times. The nuns he supervised had to do prostration on the ground. They did so many prostrations, that they wore a hollow in the earth deep enough to hide their bodies. Yeshe Losal of Samye Ling did one million prostrations. He did them on a piece of construction board so many times that it left the imprint of his hands and knees. These remarks are not directed to any of you because you are so diligent. There are other practitioners who are eager to practice the high teachings and think the ngondro is unimportant. But if you do not lay the foundation, no result will come from these practices. To quote Patrul Rinpoche, ngondro practice is more profound than the practices that follow it. By this he meant that building the proper foundation is most important. There are lots of students who try to get the high teachings like yidam practices and mahamudra and improvise many things. They try to raise the prostration board so they don't fall to the ground completely. Or use a machine that raises the board to them, so they don't need to to the prostration. Because of that I request all my students even though they take three years or more to do prostrations, that they do the traditional old methods. Even though it takes a long time, many of my students have completed ngondro in the traditional way. There are three different type of students. Two of them make good students. One type is very intelligent and take the teachings seriously and meditate seriously. The second type doesn't understand very much but follows exactly what the teacher says. But the middle type is worst. They speculate all the time and do not focus on one practice and as a result their practice is not very effective. Similarly there are three types of teachers. One type is very compassionate and realized and learned. Their only goal of presenting teachings is to guide all beings to Buddhahood. They are the most perfect type of lamas and develop a strong relation with their students. The second type of lama is very learned but their goal in teaching is to gain students for the sake of fame and possessions. Their teaching is not so effective and their relation with their students is not so pure. The third type of lama is not learned or realized and say many things without understanding or making sense except when they quote the scriptures. That would be like me. These lamas cause lots of damage.