The main thing is to not "over-clean" the leather and apply any oily conditioners, etc. You want to wipe it down with a damp cloth and make sure the dirt, dust and oils are removed on a regular basis. Aside from wiping it down like that,
Leatherique Rejuvenator Oil followed by
Leatherique Prestine Clean work really well to clean and protect the leather while also leaving a clean, matte surface.
There are two very good articles on the DI Ask-A-Pro Blog that highlight Leatherique treatments... One is the general
Leatherique Leather Care How-To and the other a more specific
Leatherique Prestine Clean Product Review.
For more info on maintaining leather and the interior, we have a good
Interior Detailing Guide on our website. Keep in mind though, even with something mild and safe like Leatherique, you don't wan to do it unless necessary. I normally try to wipe down the leather every few washes and then do a Leatherique treatment once a year or so.
Hope that helps!
Ivan @ DI