Use Saphir Rénovateur or Collonil leather cleaning spray. Myron Glaser of Glaser Designs advised me to use unscented Nivea lotion to moisturize the leather every couple of years or so.

If it is a hard, smooth, and shiny leather, you can use saddle soap to clean it a few times a year.

If it's softer and more grainy/textured, I'd probably use some of the other products mentioned - for my old soccer boots, I used mink oil fairly regularly to help with waterproofing and conditioning along with polish after every game to keep then looking sharp. I could usually get 2 or 3 good competitive seasons out of them with this maintenance routine.