When you set a mosaic it's important to be able to view it as a three dimensional thing, something you can pick up at hold in your hands, not just a two dimensional pattern. The reason for this is that you get a much better insight into the thing you are creating.
If it's a dress on a figure then see where the dress folds and which part of the folds ore nearest the viewer (the lighter parts) and which parts are in shadow because they are part covered.
In geometric mosaics look at the strands as if you can grab hold of them and pull them around. This way you understand how the strands weave around each other.
Look at this image of a three strand guilloche border, the single white centre tesserae and the triangle shaped gaps at the top and bottom filled with white tesserae aren't part of the pattern but are daylight seen through the pattern. If you held it upi to the light you would see that these are just open spaces.
(taken from 'Roman Mosaics - The Rules' the online course)