How to Clean Brass Instruments - a series of 16 short (all exactly 1' 55" each) online videos show how to flush, clean, grease, oil, and polish brass instruments (the usual demo instrument is a trumpet, but the principles are the same). Part of tutorom.com, which offers "thousands of lessons" where you may "use our tools to create... your own courses, collaborate with colleagues and more."
How to Use a Sink Hose Adapter to Clean a Horn - excellent illustrated article by Bruce Hembd
How to take your valves out - YouTube video by Ken Pope
Why Do My Valves Click? - by Bob Osmun
French Horn Maintenance - courtesy Gracewinds Music
French Horn Care & Maintenance - video courtesy PM Music Center
Horn Maintenance Video - BBC Symphony Orchestra principal Nick Korth demonstrates basic horn maintenance in this BBC video tutorial (for the first half; second half is warm-up tips).
Rotary Valve Maintenance and Stringing a Rotary Valve and Ball and Socket Actionor String Mechanism - Everything you always wanted to know about rotary valves.
How to Polish a Brass Instrument - courtesy of eHow.com
Rotary Valve Care - YouTube video courtesy of Richard Barth
Care of Your Horn - A downloadable pdf file by Alexander Mainz horn makers on horn maintenance, with a heavy emphasis on Alexander products. Has some good diagrams.
Horn Care & Maintenance - excellent illustrated article by Paxman (which also prominently mentions its products...)
Rotary Valve Maintenance - article by expert Bob Osmun. Check out the rest of www.osmun.com for all kinds of horn-related information, plus instrument and accessory sales.
Care and Maintenance of Brass Instruments - short article by Johnny Paul's Music Shop
French Horn Maintenance Manual - by Yamaha
Brass Instrument Maintenance - 40 page pdf file courtesy BIRP. Horns are p. 25-32
Band Instrument "Quick Fix" Repair Solutions - book by Gregory Biba
You might also be interested in the Horn Repair interest group on Yahoo, where you can ask (or answer!) questions on the subject.
Rotary Valve Stringing
Stringing a Rotary Valve - also by Osmun.
Stringing a Rotary Valve - downloadable pdf file by Andrew Spang
Valve Stringing - excellent article with large and clear photos by ace repairman and horn maker Stuart DeHaro. This site also offers a downloadable pdf file of the article. DeHaro also sells several kinds of rotary valve horn string, also illustrated here.
How to Re-String Rotary Valves - courtesy of Stu's Music Shop
String Action Valves - by Ricardo Matsinhos
Replacing a broken rotary valve string (silent video) - one of many C&M videos by the Band Instrument Repair Program at the State College in Red Wing, MN
Valve Stringing video courtesy of Gemstone Music
How to String a Horn Valve (YouTube Video)