None really, just let it collect dust and bring it out once or twice a year.
I’ve had it since 2019 odd and though it’s done very little mileage since then I’ve spent a fair amount of time and money on making it as nice as I possibly can, tackling all the E46 weak spots, replacing various bits of trim, wheels refinished. Lots of preventative maintenance and parts changed.
Occasionally I think about selling it but I know if I do I will likely come to regret it in years to come when they start getting rare and expensive as I probably won’t be in a position to replace it then (a bit like how an e30 325i sport is now out of my price range).