I use Auto Glym leather cleaner followed by the conditioner cream which is great.
If you need to re-colour the leather (cracks etc) I got some stuff off ebay which is a coloured cream which dyes and protects the leather. It's bloody brilliant stuff.
Used to have a 20 year old 7 series with blue leather that was all cracked and crap. Did the whole car with the stuff and it looked like new.