Once or twice every year you can apply small amounts of neutral shoe cream (like Meltonian) and polish it with soft cloth. Cream is better than wax because it nourishes leather and protects it while also giving a soft shine. Stay away from shoe polishing sponges, or leather conditioners, as they can soften the leather. If the holster gets dirty clean it with saddle soap (carefully follow the instructions). With all that said, it really depends on the leather - if it's a high quality holster made from full grain leather and dyed for color the above mentioned will work, but if it's a cheaper one coated with acrylic paint to uniform the surface and hide the defects, there really is not much you can do - cream will not penetrate the artificial top coat.