Light and shade are an important part of rendering a realistic object. Here are some tips and tricks that I've learned over the years - they may come in handy for your own artwork!
We wouldn't have shadows without light! Here are some notes you can keep in mind. When light is stronger shadows become harsher. If your light source is the sun and it's setting or rising, then shadows will be loooooong. It's also kind of fascinating that if you stand outside on a cloudy day you will hardly see any shadow at all! That's because the clouds are absorbing the light and reflecting it around you.
Shiny surfaces are tricky because they don't reflect light like normal objects. When light hits a reflective surface, the lines between values become more distinct, and there is a sharper gradation of light and dark. With typical non-reflective objects, shadows and light are somewhat softer when they transition.
More objects! Each of these items react differently to light. For example, cylinders will have a smooth gradation along it's circular surface while cubes maintain sharp contrast where two surfaces meet. When shading cloth, keep in mind there are a lot of cast shadows occurring between folds and wrinkles.