Mudguards. Carefully selected and positioned so they actually work. I see a lot of riders whose mudgurards don’t. Also those who would rather look like a coal miner than besmirch their bike’s appearance with mudguards. Bizarre. Although it turns out with a wider downtube than tyre and barely any fork clearance my gravel bike only needs a rear one.
Gaiters. Trousers if cold/wet enough, which should overlap boots. Long socks.
I find more mud and slop sits in between finned brake pads grinding away than normal ones, so ditch them.
If you’ve got several bikes, pick one to use through winter rather than trashing the whole fleet.
Fenwicks suspension lube. Amazing what can be brought back to life with that stuff. Creaking brake levers, stuck cables.
Make sure feet are warm at the start of the ride.