How to Dismantle a Valve - excellent photo guide to taking valves apart and removing rotors for cleaning, courtesy of Finke Horns.
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."
Rotary Valve Maintenance and Stringing a Rotary Valve and Ball and Socket Action or String Mechanism - Everything you always wanted to know about rotary valves.
How to String a Horn Valve (YouTube Video)
How to Polish a Brass Instrument - courtesy of eHow.com
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.
Stringing a Rotary Valve - ditto
Care and Maintenance of Brass Instruments - short article by Johnny Paul's Music Shop
First steps in making a horn - video by Finke Horns
Oiling Rotary Valves