For general leather cleaning and care, the products from Chemical Guys are about the best. If it is really dirty, you need to use Meguires D10101 all purpose Detailers cleaner concentrate diluted with water 4:1 (4 parts water) in a spray bottle.
In both cases you need a good leather cleaning brush to get into the grain and crevices and repeated cleaning may be required for tougher dirt. Chemical Guys make a nice horse hair cleaning brush that works well.
Always follow with a good leather conditioner when you are done. Both Chemical Guys and Meguirers make good ones.