Pick up a good leather jacket before you've put an arm through it and you already know. There's a weight to it that isn't heaviness exactly, more a sense that the thing has somewhere to go and isn't in a hurry. The zips move without catching, the seams sit flat under your fingers, and the leather itself has a slight resistance, not stiff, just present, the way it should be before it's learned your shape. A jacket that feels floppy and obliging straight off the hanger has nothing left to give you later. This is a piece for the in between months really, the ones either side of proper winter, when a coat is too much and a cotton jacket does nothing against the wind. It sits over a jumper in October and under nothing much at all in May, and it has no real opinion about occasions, which is its best quality. It goes to the sort of evening that starts with good intentions and ends somewhere you didn't plan, and it goes to the school run the next morning looking like it wasn't even there.
What it does over the months is the whole point of buying leather at all. The cuffs soften first, then the collar, then that crease across the back where your arms move, and none of this is damage, it's the jacket agreeing to be yours specifically rather than anyone's. Rain does it no real harm if you let it dry away from a radiator, flat or on a decent hanger, never scrunched on a hook behind a door where it'll crease in the wrong place and stay that way. A proper leather conditioner once or twice a year, worked in with a cloth rather than slapped on, keeps it from drying out and going that chalky grey at the folds that no amount of polishing fixes afterwards. Buy it too big thinking you'll grow into the slouch and you'll just look like you're wearing someone else's jacket for the whole five years it takes to decide that, so get the fit right on day one, snug across the shoulders with room for a jumper underneath, not room for a coat.
If you own none, start with black, collarless or a simple collar, nothing cropped so severely it stops being useful with trousers. Avoid anything with hardware that looks like it came off a different, cheaper jacket.
The real pleasure of it, honestly, is the sound. That particular soft creak when you shrug it on, nobody tells you that's part of what you're paying for, but it is.