Meguiars makes a 2 part cleaning/conditioning system that I find works very well.
Part 1, cleaning. Just a Aloe based cleaner. I use a microfiber applicator pad to apply. Rub it in real good, then wipe off the excess.
If your leather is real dirty, you may need to do this step twice.
Part 2, conditioner. Same technique.......rub in with a microfiber applicator, then wipe off excess.