1. Take a step back!
Sometimes viewing your artwork from a few steps back will help you identify issues you were not aware of, up close. When working from a life-model, I like standing at my easel for just this reason alone.
2. Squint, Compare, Work, Repeat.
If you are focusing on the shadow pattern one on round of squinting, focus on the light pattern in the next round. Flip-flop the negative space.
3. Don’t listen to that little Shia LeBeof voice in your head!
DO NOT “Just Do It!” A lot of people are plagued with conceptual ideas while working. They are tempted to act on them. These ideas might not make your dreams come true. Throw out what you know about facial planes, Andrew Loomis’s rules of the thumbs, and all that other brain-clutter. Think about every mark you put down and how it works towards the goal of this exercise: one cleanly defined value mass on top of your work surface.
4. Start with a size in mind.
When starting an observational work, some artists like to create a perimeter envelope for their shadow pattern to lie in - before really diving in. An envelope is useful to establish broad proportions. Do you want your apple to be the size of a penny, or the size of a paper plate?
Personally, I do not like using a fully-developed, initial envelope. Instead, I put little marks on my drawing surface to note my desired height and corresponding width. If I find these marks to be incorrect during the shadow pattern build, I can move them easily.