Cleaning and treating of leather is one that has been done to death on most car and detailing forums.
In my jaded view (!) the key is to keep the leather clean and use something like Gliptones excellent leather cleaner every 2-3 months. I just use some cleaner in a spray bottle and use a cut up sponge to apply and clean off with a fresh microfibre cloth.
If you leave the seats dirty, its not long before the dirt, grit and grime wears down the finished surface of the leather and spoils it.
The jury is out with regards to leather conditioners - that may surprise you, as it did me, but those in the leather trade say that modern leathers just need cleaning and no "conditioner" will actually do anything for the leather other than make your bum slide! The leather finish used on *modern* cars does not allow the conditioner to soak into the leather in any way thus does not do anything for the leather other than give it sheen, smell and reduce friction (oh, and possibly make you feel better for teh extra cash you just spent!).
If your leather has worn badly, I suggest getting some leather dye from the likes of
Dynamix - just tell Ryan what colour you need and he will ship it pronto - great service and product.
Hope that helps.
David