If you don't want seat covers, the key is regular maintenance. That means cleaning the seats once a week or so with a damp towel and then using a leather cleaner less often. Really depends on how much time you have and how dirty they get, but you could stretch to once a month. There are car specific cleaners and protectors which I'll link to below, or you can use something like Leatherique which we hear good things about. Once a year you'll want to apply some kind of leather oil or the like just to keep the seats soft and glossy. You can have a look at what you think will work for you. Essentially, it's just about keeping on top of it and not letting the leather dry out and crack, or get discoloured.

Meguiar's 3-in-1 leather cleaner: amzn.to/3gerH48

Autoglym leather cleaner: amzn.to/33fsyhr

Leatherique oil: amzn.to/39G585L