Difficulty: Moderately Easy
Step1
If the stain is from pizza, body oils, or other any other greasy substance, you should use a de-greaser first, then clean with a mild soap and finally, condition. Furniture Stuff makes a leather stain degreaser as well as an interesting line of products that work really well. For suede, the procedure to remove a stain is the same except that you'll use a cloth especially made for suede leather.
Step2
If the leather is pigmented, you can use any leather cleaner to remove the stain. Keep in mind that spot cleaning a leather stain is always difficult because water and chemicals react with it differently.
Step3
Of course the best approach to dealing with any leather stain, especially leather furniture is to prevent it. If you eat or drink on your leather furniture sit forward, if you are wearing a leather garment while eating, protect it!