How do I properly maintain a mechanical metronome?

One thing people often forget: before putting it away, let it sit relaxed for a few days rather than storing it with the spring fully wound up. That puts way less strain on the mechanism over time.

You're hitting on the right points, but it's not just storage that matters - what really kills a metronome is when the pendulum gets misaligned or you force it. I'd say the most important thing is never to wind the spring beyond its natural resistance point, or you'll end up snapping the internal gear. And then regularly check that the pendulum swings completely free without rubbing against the casing, because even the slightest friction will wear down the mechanism over time.

The real key is regularity - use it regularly rather than letting it sit idle for months, that keeps the spring and gears in good shape. The basics (dry, covered, don't force the pendulum) are obvious, but a lot of people forget that letting the mechanism run from time to time is actually better for its longevity than perfect storage but sitting unused for a year.

Don't forget that maintaining a mechanical metronome isn't just about storage - the really critical thing is to avoid forcing the spring when you wind it up or shaking the device. People often think just running it regularly is enough, but honestly if you handle it badly while you're using it you can screw up the pendulum's balance in just a few months.

That said, what people say about releasing the spring before you store it is solid advice - just add that an occasional check on the pendulum's position (making sure it stays properly vertical and has no side play) is really worth doing, especially if your metronome is a few years old.