The most important part of the layering is making sure the dressing and the spinach (or whatever 'leaf' you choose) don't touch. As long as they stay separate, these salads can be made up to 4 days in advance and will stay fresh in the fridge with a lid on. This is one trick to eating salad every day for lunch at work. You can make a few jars on Sunday night and just grab one to bring to work daily. When you're ready to eat, just shake it up.