See also Practicing, Basics, Low Range
Some Random Ideas and Reminders for Improving Accuracy by Nicholas Smith
Eight Ways to Improve Your Accuracy - by John Ericson
Tips and Tricks for Improving Accuracy on the Horn by James Boldin (pdf)
Vanishing Technique - Pip Eastop
Extended Techniques - stunning demonstration by the initimable Dani Reynolds as only he can do.... a MUST SEE!
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
15 Second Sight Reading Routine - blog article by Jeffrey Agrell
Strategies for Sight-Reading and Improvising Music - Ch. 8 from Music Excellence, by Sam Thompson and Andreas C. Lehmann
How to prepare an Etude - article (pdf) by David Vining
Sometimes it’s fun and refreshing to play music that was not written for horn (after all, we do play a lot of transcriptions...). There is a wealth of fiddle tune music that is both challenging and fun and will really hone your arpeggios in sharp keys. O’Neill’s Music of Ireland is one such source, available here in various formats. There are 8 sections of tune types; probably the ones to try first would be the reels and the hornpipes. Since it’s fiddle music, it often goes quite high. No matter: it’s a chance to work on transposition as well.