I have a 2-drawer filing cabinet. Top drawer is alphabetical by composer, bottom is collections, alphabetical by title. In between them are my altered harmony books.

I was unclear as to whether this is for personal or church; organ or choral. I was referring to my own organ music.

For church, I've usually had a cataloged system, with music simply put away in order of first-purchased to most recent.

Yes, that's what my last church had, namely in several binder indexes. I would have preferred moving to a database, but that would have been a large project.

Alphabetical by title works most of the time. Around Christmas and Easter, the system is pile A, pile B, and so forth. Don't forget misc.

Previous church had typical tall filing cabinet, basically organized alphabetically.
Next church (I was only temporarily there for a few months) had bookshelf-type things on the walls, and it was arranged by Mass/season then alphabetically. I found that confusing when trying to find a piece, but I definitely see advantages and disadvantages to both systems.
Current church has an 80+year old dusty, dirty, somewhat-alphabetized, photocopied mess, lol!

What library ? LOL

We have two filing cabinets of masses (and really need a third to start on). These are by composer, with the anonymous ones at the beginning, and individual mass parts not part of cyclical masses preceding those under Kyrie, Gloria, etc.

The motets are arranged differently in four filing cabinets, but they are by the first word of the text. Since we sing the Tridentine mass the composer is not really important but the text is, and we have found that this works best. There is a drawer and a half of extra chant pieces (i.e. things that are either not in everybody's Liber or that have been collected on sheets so people aren't flipping around and wasting time during the mass or office). For example, the propers for yesterday would be found in the same order as in the Liber (temporale, 16th Sunday after Pentecost), but the sheets have the Gradual printed on them in the right order for ease of use (in the Liber you have to go back and find Timebunt gentes somewhere else). There are some oversize things that can fit lengthwise in a drawer.

(BTW, I also keep one copy of everything at home so I can make more copies/check on music and do some groundwork without traveling to the church/advise people over the phone, etc.)

Keeping this all under control is quite a job, and I have a computer list, but updating it is a mess unless you really keep on it, and everything is filed immediately after use so that you can "read" the drawers like they read bookshelves in a library.

The best thing is having things alphabetized by composer (for masses) or text (for motets) so you can at least go there and find something.

Yes Mara, that is similar to what I have, but perhaps a little more organized. I need to get on top of this, in my "spare" time.

One of the simplest and most clever things I've seen from an avoiding-page-turning perspective was at the CMAA chant workshop in Houston. The organist at the co-Cathedral used a giant spiral bound sketchpad into which she was able to tape 4 pages of music onto each page. This gave her a spread of 8 pages without a page turn, and kept loose-leaves from flying off the music stand.

Hmmm. I need to look for one of those sketchpads! Hadn't thought of that. Eight pages would cover most compositions, and reducing the size of the pages would allow pasting in a couple more pages. Good idea!