Well, one thing I've tried on some figures is to (after I prime it), set it under a strong overhead light coming in almost directly above (you can angle it slightly from the front). I then take pictures of the model from different angles (front, sides, back, and more if I want) while always keeping the light above and slightly in front (between me and the model). You can then use these pictures as a guide for where your highlights and shadows should fall. You'll still want to take some liberties though. For example, a hat might keep the figures face in shadow but you'll still want to highlight the nose, cheeks, etc. But I've found this option very helpful for shading folds in clothes and other complex shapes.
If the model is metal, I recommend priming it before taking the pictures. You want a single color figure that isn't reflective. If you've already base coated a figure but haven't done highlights or shadows, that would work as well (so all red areas are a single shade of red, all white areas just white, etc). But if possible I would do this before I started to add any color to the figure.
This isn't the only way, but I've found it to be helpful.