This may seem like a small point but I find it to be important; the difference between positive and negative corrections. Whenever possible I try to tell students what they should be doing rather than what they're doing wrong, Consider the corrections "don't drop your hands" vs. "keep your hands up". They both mean essentially the same thing but on a subtle level the latter statement gives the student something to work towards while the former reminds him/her of a mistake.
There is definitely a time and a place for negative corrections, violations of school protocol for example. But overall I've found that teaching students what to strive for gives them a better understanding of their progress. This is especially true for teenagers who I find don't generally react well to constant criticism.
This idea is a break from a lot of traditional martial arts styles. Many old school teachers are stern and punitive in their method. While I keep my training sessions tough I find positive corrections to be a better fit for modern western students.
Traditional Chinese martial arts for mind/body development