Horn
Technique
See also Practicing, Basics
Some Random Ideas and Reminders for Improving Accuracy by Nicholas Smith
Eight Ways to Improve Your Accuracy - by John Ericson
Vanishing Technique - Pip Eastop
Building Endurance - very short but very to the point answer to a question by Phil Myers, who knows whereof he speaks
Video - Rex Richardson Basic Flexibility for Brass Players - Rex is trumpet player, but the flexibility exercises he demonstrates are good for all brasses.
Horn Techniques - short Wikipedia article; see also this one with brief links to sections on posture, right hand, fingering chart, breathing, embouchure, articulation, dynamics, mutes, reading music, tips & tricks, and transposition.
Horn Technique - covers a variety of topics, part of "Horn Player's 'Where to Find'"
The Standley Routine - description of Forrest Standley's system of scales and arpeggios.
Vibrato - by Thomas Bacon
Power Scales - by Jeffrey Agrell (pdf download). Learning shorter, faster scales.
The Art of Sight Reading - blog article by Jonathan West
Strategies for Sight-Reading and Improvising Music - Ch. 8 from Music Excellence, by Sam Thompson and Andreas C. Lehmann