Maybe EVERYBODY is right!
Once upon a time, I used to think there was one way about going about the business of trombone playing. As I gain more experience playing and teaching, I realize how many different ways there are to help someone (including myself) be successful. The fact of the matter is, most pedagogical information we receive is based in relativity, and you can almost always go too far in one direction or another. Perhaps a teacher tells you to practice moving your slide slower and with a greater sense of control. Chances are, if he/she is a knowledgeable teacher, the teacher is right. But how far do you take this? There is a point where you can move the slide too slow. Then you go to someone else and they give you opposite information. They are telling you the truth too! For someone who is not completely aware of their own tendencies and acutely aware of what sounds good and doesn't, this can be quite confusing. At the end of the day, both teachers are right. It's up to you to put in the hours isolated in a room to figure out the appropriate balance.
I think a smart practicer has a healthy sense of adventure and willingness to experiment. If you are willing to practice opposite extremes, even when they go too far, you'll have a better recognition of when things are correct and how you got to that point. Take clear articulations as an example. Many people have difficulty getting a clear start because the air is too slow and late getting to the instrument after the seal of the tongue is broken. To remedy this issue, most would beat their head against the wall trying to get the air as close behind the release of the tongue as possible. I think a more innovative way to go about it would be to blow air through the horn with no tongue and then articulate once the air stream is moving. In effect, you're getting the air to be not just even with the release of the tongue, but ahead of it. Then work backwards from that point, gradually bringing the tonguing motion closer to the front of the air stream, until you get them to line up exactly where you want. You might be surprised at how well this works. Practice that every day for 10 minutes for a few weeks and you will be happy with the results. I'm sure many of you have used the idea of glissing or flutter tonguing through a passage to make sure the air is consistently moving forward. Why not be as extreme and innovative in finding solutions in other aspects of playing?