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Lots of brass instruments that were playable when "put away" emerge years later with everything stuck.
What preparations do you recommend for long-term storage of brass instruments?

Here's my guess on general preparations for a piston-valved instrument:
1) Remove everything from the instrument body and from each piston
2) Clean the inside and outside of the instrument. (With what?)
3) Dry the outside and turn the instrument body to allow pooled water to escape
4) Let the inside air-dry for a few days.
5) Store the disassembled pieces: mouthpiece, buttons, valve stems, valve caps, guides, piston felts/corks, pistons, springs & slides in a sturdy box that will not deteriorate.
6) Store the instrument body in its case. Attach the sturdy box of pieces to the case.

When ready to re-assemble:
1) Clean and lubricate all the boxed pieces including threads on buttons, stems & valve caps.
2) Clean and lubricate the valve casings (including threads) and the inside tubing for the slides.
3) Reassemble

Or you could send it to me and I'd play it every day...

At the end of every school year I flush out all of the brass instruments with water and then heavily lubricate the valves. I have been doing this for a long time. This year after my annual plea for student to move to brass instruments other than trumpet or trombone (the recruitment process in my district sucks) I was particularly successful and found I am using all of the French horns in my inventory. Some of these horns haven't been played for over 8 years. They all worked great, not a single stuck valve!

As a side note, I now have 22 trumpets, 4 horns, 8 trombones, 3 euphs and two tubas. Not too shabby! Now to get them playing.