Scott Morris gives a lesson on Barring Technique (the Barre). This comes via the excellent GSI (Guitar Salon International), thanks! Their blurb: “In this lesson Scott addresses the topic of barring – when and how to bar, and also when to use a half bar or not to bar at all. Scott is playing a 1999 Gioachino Giussani classical guitar at the Guitar Salon International showroom in Santa Monica, CA.” This comes from his method Classical Guitar Complete. Interior/exterior barres also know as pivot barres if you’re using RCM or other material. If you’re looking for a technique book you can check out my review of classical guitar technique and exercise books.
One extra tip might be regarding problem solving with the barre. If a string is muted in your barre you need to fix something instead of applying more pressure to the whole barre. Check if you’re close to the fret and on the side of your finger as Scott suggests. But then you need to zero-in on the string that is the problem and isolate it. Then move that very spot on your finger to apply a small amount of functional tension to that exact spot that contacts the problem string. So I’m talking about micro movements within the length of your finger. For example, if the 3rd string is buzzing you need to apply a subtle amount of push to the spot on your finger that contacts the 3rd string (not the spot that contacts the 2nd or 4th string but the exact spot of the 3rd string). Obviously it takes some time to develop this subtle dexterity within your finger.