Lighting is a very complicated issue, so 3D programs separate it into special "maps", each dealing with a single aspect of light. We can use this technique in Adobe Photoshop, but instead of painting the character as a whole, we can create a series of layers based on 3D maps. The famous Ambient Occlusion is one of them. This is a universal technique, and while it's not fast, it lets you understand lighting so deeply that you'll be able to develop other, faster methods afterwards, or to find out what's wrong with the method you're currently using.